UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


HEREDITY  AND    EVOLUTION 
IN   PLANTS 


GAGER 


FRONTISPIECE 


Restoration  of  a  scene  along  a  sluggish  creek  in  Texas  and  New 
Mexico  during  the  late  Carboniferous  (Upper  Pennsylvanian)  and  early 
Permian  times.  The  lowlands  of  this  period  doubtless  swarmed  with 
reptiles  such  as  shown  in  the  picture,  and  with  other  animals,  now 
extinct.  Some  specimens  of  the  giant  "dragon-flies"  had  a  spread  of 
wings  of  two  feet.  The  fern-like  trees  and  the  bushy  plants  in  the  fore- 
ground are  Cycadofilicales.  To  the  right  of  the  water  are  wide  stretches 
of  the  huge  scouring  rush  (Catamites);  on  the  left  bank  of  the  stream  are 
the  unbranched  Sigillarias  (still  as  prominent  as  earlier  in  the  coal 
period),  and  on  higher  ground  to  the  left  the  branched  Lepidodendrons. 
One  must  view  this  scene  as  one  of  many  such  landscapes,  with  ever- 
varying  detail,  along  streams  and  inlets.  Cordaites,  which  in  later 
Devonian  time  made  the  first  great  forests  of  which  there  is  record,  is 
still  present,  though  not  shown.  So,  too,  there  are  hidden  in  the  recesses 
of  the  forest  the  forerunners  of  the  modern  coniferous  types,  as  well  as 
other  forms  destined  to  give  rise  to  the  angiosperms.  (Landscape  from 
Williston,  adapted  from  Neumayr.) 


HEREDITY 

AND 

EVOLUTION    IN    PLANTS 


BY 

C.  STUART  GAGER 

DIRECTOR    OF   THE   BROOKLYN   BOTANIC   GARDEN 


WITH  113  ILLUSTRATIONS 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012  WALNUT  STREET 
1920 


COPYRIGHT,  1920,  BY  P.  BLAKISTON'S  SON  &  Co. 


To  the  Memory  of 
BENJAMIN  STUART  GAGER 


"  What  a  science   Natural   History    will    be 
when  .     .     .  all  the  laws  of  change  are  thought 
one  of  the  most  important  parts  of  Natural 
History." — 
Charles  Darwin.    (Letter  to  J.  D.  Hooker.) 


PREFACE 


The  present  little  book  was  originally  intended  to  be 
merely  a  reprint  of  Chapters  XXXI  to  XXXVIII  of  the 
author's  Fundamentals  of  Botany.  The  reprinting  of  those 
chapters  was  suggested  by  comments  received  from  various 
correspondents,  who  pointed  out  that  the  subject  matter 
which  they  cover  had  not  been  elsewhere  presented  in  so 
concise  a  treatment  in  one  volume,  and  in  a  manner 
suited,  not  only  to  beginning  students,  but  also  to  more 
general  readers.  The  chapter  on  Experimental  Evolu- 
tion has  received  the  approval  of  the  author  of  the  mu- 
tation theory,  as  being  an  accurate  presentation  of  the 
essentials  of  that  theory.  "  I  have  especially  appreci- 
ated," writes  Professor  de  Vries,  "the  statement  of  the 
difference  between  fluctuating  and  saltative  variation, 
which  is,  to  my  mind,  the  real  empirical  basis  for  the 
theory,  far  more  than  the  experiments  on  mutation  with 
single  plants.  The  relation  of  my  view  to  Darwinism 
is  misunderstood  by  many  authors,  and  it  is  a  great  satis- 
faction to  me  that  you  have  outlined  it  in  such  a  plain 
way." 

In  the  preparation  of  the  copy  for  reprinting,  consider- 
able new  matter  has  been  added,  certain  sentences  and 
paragraphs,  pertinent  only  to  an  elementary  text-book, 
have  been  omitted,  and  others  recast,  and  several  fresh 
illustrations  have  been  introduced,  either  as  new  or  as 
substitutes. 


Chapters  X,  Geographical  Distribution,  and  XIII, 
The  Great  Groups  of  Plants,  and  the  Bibliography  are 
new.  No  attempt  has  been  made  to  cite  the  voluminous 
periodical  literature  in  the  Bibliography,  but  needless  to 
state,  this  has  been  freely  consulted  and  drawn  upon. 
Numerous  citations  are  given  as  foot-notes,  especially  in 
Chapter  X. 

In  going  over  the  chapters  it  also  became  evident  that 
since,  in  order  to  read  them  understandingly,  one  must 
have  a  clear  conception  of  the  facts  of  the  lift  history  of  a 
vascular  plant,  it  would  be  best  to  introduce  from  the 
Fundamentals  of  Botany  the  three  chapters  (viz.  XII- 
XIV)  on  the  life  history  of  the  fern.  As  stated  in  the 
Preface  to  that  hook,  while  the  ultimate  problem  of  botany 
is  the  development  of  the  kingdom  of  plants,  the  more 
immediate  and  fundamental  problem  is  the  development 
of  the  individual  plant.  "Ontogeny  is  fundamental 
because  without  a  knowledge  of  its  processes  the  processes 
of  phylogeny  cannot  be  comprehended.  Phylogeny  is 
the  ultimate  problem  because  its  complete  solution  in- 
volves an  orderly  description  of  all  the  phenomena  of 
plant  life,  and  their  relation  to  each  other." 

The  author  is  specially  indebted  to  Dr.  O.  E.  White, 
curator  of  plant  breeding  in  the  Brooklyn  Botanic  Garden, 
for  a  careful  reading  of  the  entire  manuscript  and  for 
many  valuable  suggestions;  also  to  Mr.  Norman  Taylor, 
curator  of  plants,  in  connection  with  Chapter  X,  and 
to  Dr.  Alfred  Gundersen,  associate  curator  of  plants  in 
the  same  institution,  for  numerous  constructive  criticisms 
in  connection  with  Chapter  XIII.  The  diagram  show- 
ing the  apparent  affinities  and  approximate  geological 
distribution  of  the  main  groups  of  vascular  plants  (p. 


248)  originated  with  Dr.  Gundersen,  but  has  been  modi- 
fied, as  here  printed,  in  certain  details  for  which  the 
author  alone  is  responsible.  Grateful  acknowledgment  is 
made  to  Dr.  Ralph  E.  Cleland  for  photo  prints  of  Figs. 
74  and  75  from  negatives  made  by  him  on  the  summit 
of  Mt.  Madison  (Adirondacks) ;  and  to  Prof.  Harvey 
W.  Shimer,  author,  and  The  Macmillan  Co.,  publisher, 
for  permission  to  reproduce  Fig.  66. 

If  the  following  pages  shall  prove  to  be  a  source  of  re- 
liable and  readable  elementary  information  to  those  in- 
terested in  the  subjects  treated,  the  object  of  the  book 
will  be  accomplished. 

BROOKLYN  BOTANIC   GARDEN, 
March  25,  1920. 

C.  STUART  GAGER. 


CONTENTS 


CHAPTER  .       PAGE 

I.  LIFE  HISTORY  OF  A  FERN i 

II.  LIFE  HISTORY  OF  A  FERN  (Concluded)      20 

ill.  FUNDAMENTAL  PRINCIPLES 31 

IV.  HEREDITY 45 

V.  EXPERIMENTAL  STUDY  OF  HEREDITY .    .     55 

VI.  EVOLUTION 79 

VL1.  DARWINISM 90 

VIII.   EXPERIMENTAL  EVOLUTION 101 

IX.  THE  EVOLUTION  OF  PLANTS 124 

X.  GEOGRAPHICAL  DISTRIBUTION 139 

XI.  PALEOBOTANY 183 

XII.  THE  EVOLUTION  OF  PLANTS  (Concluded)      201 

XIII.  THE  GREAT  GROUPS  OF  PLANTS 243 

BIBLIOGRAPHY 252 

INDEX 257 


HEREDITY  AND  EVOLUTION 
IN  PLANTS 


CHAPTER  I 
LIFE  HISTORY  OF  A  FERN 

1.  Life  History. — Every  plant,  in  the  course  of  its  ex- 
istence,  passes   through  a  series  of  changes  in  orderly 


FIG.  i. — A  fern  (Anisosorus  hirsulus),  showing  portion  of  the  stem  above 
ground. 

sequence.  Like  an  animal,  every  plant  begins  life  as  a 
single  cell,  the  egg,  or  the  equivalent  of  an  egg.  Except 
in  some  of  the  lower  forms,  the  egg  develops  into  an 


HEREDITY  AND    EVOLUTION  IN  PLANTS 


FIG.  2. — Portion  of  the  rhizome   of  the  common  brake  (Ptcris  aqnilina) 
showing  a  cross-section  view  at  the  right. 


FIG.  3.— Cross-section  of  the  rhizome  of  the  bracken  fern  (Ptcris  aqni- 
lina), showing  the  tissue  systems.     Greatly  magnified. 


LIFE   HISTORY   OF   A   FERN  3 

embryo,  and  the  embryo  matures  into  an  adult.  By  a 
series  of  more  or  less  complicated  processes  the  adult 
eventually  gives  rise  to  another  egg,  like  the  one  from 
which  it  came,  thus  completing  one  life-cycle  and  initiat- 
ing another.  These  various  changes  constitute  the  life 


FIG.  4. — Tree  ferns  on  the  military  road   between  Cayey  and  Caguas, 
Porto  Rico.     (Photo  by  M.  A.  Howe.) 

history  of  the  individual.  The  various  stages  of  life 
history  common  to  most  plants  are  nowhere  more  clearly 
illustrated  than  in  the  ferns. 

2.  Description  of  a  Fern  Plant. — The  more  common 
ferns  of  temperate  regions  have  underground  stems  or 
rhizomes  (sometimes  called  root-stocks),  so  that  only  the 


4  HEREDITY   AND   EVOLUTION   IN   PLANTS 

leaves  appear  above  ground.1  The  stem  may  be  branched 
or  unbranched.  When  branched,  the  branches  are  pro- 
duced without  reference  to  the  insertion  of  the  leaves, 
in  contrast  to  the  habit  of  higher  plants  of  forming 
branches  only  in  the  upper  angle  (axil)  between  the  stem 
and  leaf-stalk.  There  is  always  a  terminal  bud  at  the 


FIG.  5. — A,  Upper  epidermis;  B,  lower  epidermis  of  the  leaf  of  the  fern, 
Drynaria  meyeniana.     (Camera  lucida  drawing.) 

tip  of  the  fern-stem  (and  of  the  branches  when  any  oc- 
cur); and  the  leaves  are  usually  attached  just  back  of  this 
tip.  The  stems  are  commonly  (though  not  always) 
covered  by  hairs  or  scales  (Fig.  i),  and  on  their  older 
portions,  at  some  distance  back  from  the  tip,  may  be  seen 
the  scars,  or  the  ends  of  leaf-stalks,  left  by  old  leaves  that 
lfThe  leaves  of  ferns  are  often  called  fronds. 


LIFE   HISTORY    OF   A   FF.RN 


have  died  and  fallen  away.  The  rhizome  bears  true  roots 
(Fig.  2),  and  its  tissues  are  differentiated  into  epider- 
mal, fundamental,  mechanical,  and  conducting  systems 
(Fig.  3).  In  tropical  countries  there  are  "tree  ferns," 


FIG.  6. — Osmunda  Claytoniana.    Young  sporophylls,  showing  circinate 
vernation.     Note  the  spore-bearing  pinnae. 

with  upright  stems,  and  this  type  of  fern  is  common 
among  the  fossil  plants  of  earlier  geological  ages  (Fig.  4). 
There  are  also  climbing  ferns. 

3.  Two  Kinds  of  Fern-leaves. — Careful  examination 
of  the  leaves  of  certain  mature  ferns  will  disclose  the  fact 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


that  they  are  not  all  alike.  Some  of  them  are  merely 
foliage-leaves,  and  do  not  differ  in  any  essential  point  from 
the  foliage-leaves  of  higher  plants,  such  as  the  maple  or 
lily;  they  possess  stomata  for  the  exchange  of  gases  and 


FIG.  7. — Portions  of  the  sporophylls  of  two  ferns  to  show  the  sori. 
On  the  left  Poly podium  punctatum  (L.)  Sw.;  on  the  right  a  variety  of  Pteris 
longifolia,  with  sporangia  marginal  on  the  pinnules. 

water-vapor  with  the  outer  air  (Fig.  5),  and  they  also 
resemble  the  leaves  of  higher  plants  in  their  internal  struc- 
ture. All  fern-leaves,  however,  have  a  very  characteristic 
arrangement  in  the  embryonic  or  bud  condition,  being 


LIFE   HISTORY   OF   A   FERN 


coiled  up  from  the  tip.  As  the  leaves  grow  they  unroll; 
and  in  some  ferns,  at  certain  stages,  they  often  closely  re- 
semble the  neck  of  a  violin  (Fig.  6).  The  leaf -blade 


FIG.  8.— Sporophylls  of  two  ferns.     At  the  left,  a  species  of  Poly  podium 
(Phymatodes),  having  no  indusium;  at  the  right,  Diplazium  zelanicum. 

possesses  veins  of  fibro-vascular  bundles  that  pass  down 
the  leaf-stalk  and  through  the  stem  to  the  roots.  Because 
of  the  possession  of  these  vascular  bundles,  ferns  (and 
all  other  plants  of  which  this  is  true)  are  called  vas- 


HEREDITY   AND   EVOLUTION   IN   PLANTS 

cular  plants.  These  leaves  perform  all  the  functions 
performed  by  the  foliage-leaves  of  other  plants,  the  most 
important  of  which  are  the  manufacture  of  organic,  car- 
bohydrate food  from  inorganic  raw  materials  (photosyn- 
thesis}, and  the  giving  off  of  water  vapor  from  within 
(transpiration) . 

4.  Spore-bearing  Leaves.— The  second  type  of  fern- 
leaf  bears,  on  its  underside,  numerous  " fruit-dots''  or  sori 
(singular  sorus)  (Figs.  7  and  8).  These  structures  have 
to  do  with  reproduction.  A  single  sorus  of  such  a  fern 


FIG.  9. — Cross-section  through  the  marginal  sorus  of  a  sporophyll  of 
the  bracken  fern  (Pteris  aquilina).  I,  palisade  layer;  fb,  vascular  bundle; 
sp,  sporangium;  in,  indusium.  (Greatly  magnified.) 

as,  for  example,  Polypodium,  is  composed  of  a  cluster  of 
tiny  stalked  cases.  The  cases  contain  minute  unicellular 
reproductive  bodies  called  spores,  and  the  entire  structure 
is  a  sporangium.  The  place  where  the  sporangia  are 
attached  to  the  leaf  is  the  sporangiophore1  (Fig.  9),  and 
over  all  is  often  found  a  thin  membranous  covering,  the 
indusium  (Figs.  9  and  10).  In  some  ferns  the  indusium 
is  lacking,  and  the  sorus  is  naked.  Spore-bearing  leaves 
are  called  sporophylls,  and  plants  that  bear  sporophylls 
are  called  sporophytes. 

lAlso  called  receptacle. 


LIFE   HISTORY    OF   A   FERN 


FIG.  10. — Cyrtomium  fakatum.     Under  (dorsal)  surface  of  a  portion  of 
a  sporophyll,  showing  the  numerous  sori  on  the  pinnae. 


FIG.  ii.— Fern  leaves,  showing  various  degrees  of  subdivision  or  brandl- 
ing of  the  blade.     .4,  Phyttilis;  B,  Poly  podium;  C,  Pteris;   D\  Adiantum. 


10  HEREDITY   AND   EVOLUTION   IN  PLANTS 

5.  Types  of  Foliage  -leaf.— In  some  ferns  the  foliage- 
leaf  presents  a  simple,  unbranched  blade,  and  petiole; 
but  in  other  species  the  blade  is  variously  branched.     In 
such  cases  the  larger,  primary  divisions  are  called  pinnce, 
and  the  secondary  subdivisions  pinnules.     Illustrations 
of  these  various  types  are  shown  in  Fig.  1 1 . 

6.  Sporangia. — As  noted  above,  each  sporangium  con- 
sists of  a  spore-case  borne  on  a  stalk  (Fig.  12).     The  struc- 
ture of  the  case  varies  considerably  in  various  groups  of 
ferns,  but  it  usually  possesses  walls  only  one  cell  thick,  with 
a  clearly  differentiated  region,  the  annulus,  composed  of 
cells  whose  radial  and  inner  cell-walls  are  greatly  thick- 
ened.    Various   types   of  spore-cases   are  illustrated  in 


FIG.  12. — Sporangia  of  an  undetermined  species  of  fern;  /«,  lip-cells; 
an,  annulus;  st,  stalk;  sp,  mature  spores.  Each  of  the  four  nuclei  in  the 
upper  cells  of  the  stalk  is  in  the  terminal  cell  of  one  of  the  four  vertical 
rows  of  cells  that  compose  the  stalk. 

Fig.  13.  Among  the  group  of  ferns  which  are  now  most 
common,  and  to  which  the  bracken  fern  (or  "brake"), 
the  maiden-hair  fern,  the  common  polypody,  and  others 
belong,  the  sporangium  always  originates  from  a  single 
epidermal  cell.  Ferns  whose  sporangia  thus  originate  are 
called  leptosporangiate  ferns  (Cf.  p.  29).  The  walls  of 
their  spore-cases  are  always  only  one  cell  thick,  and 


LIFE   HISTORY   OF   A   FERN 


always  possess  some  form  of  annulus.  As  the  sporangia 
mature  the  spore-case  itself  becomes  differentiated  into 
two  distinct  kinds  of  tissue,  namely,  vegetative  tissue  on 
the  outside,  forming  the  wall  and  reproductive  tissue  within, 
from  which  the  spores  are  developed. 

7.  Number  of  Spores. — The  number  of  spores  pro- 
duced by  a  vigorous  fern  is  a  great  revelation  to  one  who 
has  never  given-  such  matters  careful  thought.  Pro- 


FIG.  13.  —  Types  of  fern  sporangia.  A,  Loxsoma  Cunninghami;  B, 
Gleichcnia  circinata;  C,  Todea  barbara;  D,  Thyrsopteris  elegans;  E,  Malonia 
peclinata;  F,  Lygodium  japonic-urn.  (Redrawn  from  various  sources.) 

fessor  Bower,  of  Glasgow,  has  called  attention  to  this  fact 
following  words: 


estimate  may  be  made  of  the  numerical  output  of  spores  from 
a  large  plant  of  the  Shield  fern,  as  follows:  In  each  sporangium  48' 
spores  may  be  formed;  a  sorus  will  consist  of  fully  100  sporangia,  usually 
more;  20  is  a  moderate  estimate  of  the  sori  on  an  average  pinna;  there  may 
be  fully  50  fertile  pinnse  on  one  well-developed  leaf,  and  a  strong  plant 
would  bear  10  fertile  leaves.  48  X  ico  X  20  X  50  X  10  =  48,000,000. 
The  output  of  spores  on  a  strong  plant  in  the  single  season  will  thus,  on  a 
moderate  estimate,  approach  the  enormous  number  of  fifty  millions." 

8.  Types  of  Sporophylls  —  In  many  ferns  the  leaves 
serve  both  vegetative  and  reproductive  functions  in  about 

1  Bower  gives  this  number  as  the  characteristic  output  for  the  species 
Aspidium  Filix-mas.     In  other  species  the  number  may  be  64. 


1 2  HEREDITY    AND    EVOLUTION   IN   PLANTS 

equal  degree,  as  in  the  case  of  Polypodium  mentioned 
above.  In  some  species,  however,  there  are  two  kinds  of 
leaves — one  devoted  entirely  to  vegetative  functions,  and 
another  to  the  reproductive,  or  spore-producing  function 
(Fig.  14);  between  these  two  extremes  all  grades  of  transi- 
tion are  found  (Fig.  15).  But  however  widely  the  sporo- 


FlG.  14. — The  cinnamon  fern   (Os  HI  undo,  cinnamomca),  showing    foliage 
leaves  and  sporophylls. 

phyll  departs  from  a  foliage-leaf  in  appearance,  it  must, 
nevertheless,  be  regarded  as  morphologically  a  leaf.  As 
partial  evidence  of  the  true  foliar  nature  of  sporophylls, 
there  may  be  cited  the  interesting  experiment  of  Atkinson, 
who,  by  removing  the  true  foliage-leaves  just  beginning  to 
unfold  in  the  spring,  was  able  to  induce  developing  sporo- 
phylls to  alter  their  character,  and  become  transformed 


LIFE   HISTORY   OF   A   FERN 


FTG.  15. — Clayton's  fern  (Osmunda  Claytoniana) ,  showing  sporophylls 
in  the  center,  surrounded  by  foliage  leaves. 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


into  foliage-leaves.  Similar  results  were  also  obtained  by 
Goebel.  These  experiments  indicate  that  foliage-leaves 
and  sporophylls  are  very  closely  "related  to  each  other, 


FIG.  16. — Portion  of  a  leaf  of  a  fern  (Tedoria  cicutaria)  that  bears 
bulbils  on  both  the  upper  and  lower  surfaces  of  its  leaves.  Plantlets 
develop  from  the  bulbils  while  they  are  still  attached. 

and  demonstrate  clearly  that  foliage-leaves  may  be  pro- 
duced by  the  sterilization  of  spore-bearing  leaves.  The 
interesting  question  here  naturally  arises  as  to  whether,  in 
the  evolutionary  development  of  the  plant  kingdom 


LIFE   HISTORY   OF   A   FERN 


FIG.  17. — Walking  fern  (Camptosorus  rhizophyllus).  The  smaller, 
lower  plant  originated  at  the  tip  of  a  leaf  of  the  larger  plant,  and  one  of  its 
leaves  has,  in  turn,  struck  root. 


10  HEREDITY   AND    EVOLUTION   IN   PLANTS 

through  long  geological  ages,  foliage-leaves  have  in  gen- 
eral originated  by  the  sterilization  of  spore-bearing  organs. 
9.  Vegetative  Multiplication. — In  addition  to  repro- 
duction by  spores,  ferns  may  also  be  propagated  vege- 
tatively  in  at  least  four  ways.  By  one  of  these  methods, 
the  rhizome  is  cut  into  several  pieces,  and  from  every 
piece  that  contains  a  bud  a  new  plant  will  develop.  By 


FIG.  1 8. — A  Boston  fern  (Ncphrolepis),  reproducing  vegetatively  by 
means  of  runners  or  stolons.  The  parent  plant  is  in  the  round  pot. 
(After  R.  C.  Benedict.) 

the  second  method,  the  plant  is  progagated  by  means  of 
bulbils,  which  occur  on  the  foliage-leaves  of  several  species. 
In  the  fern  Tectoria  cicutaria,  bulbils  occur  on  both  the 
upper  and  under  sides  of  the  leaf  (Fig.  16).  These  bulbils 
fall  to  the  ground,  and  under  suitable  conditions  of  light, 
moisture,  and  temperature  give  rise  to  new  fern-plants. 
One  of  the  ferns  native  to  the  eastern  United  States 
(Cystopteris  bulbifera)  received  its  specific  name  from  the 


LIFE   HISTORY   OF   A   FERN  17 

fact  that  it  bears  bulbils.  A  third  method  is  illustrated 
in  the  very  interesting  "walking  fern"  (Camptosorus 
rhizophyllus),  where  the  tips  of  the  long  acuminate  leaves 
rest  upon  the  moist  ground,  take  root,  and  develop  an 
entire  new  plant  at  the  distance  of  the  leaf's  length  from 
the  parent  fern  (Fig.  17).  The  result  of  several  repeti- 
tions of  this  suggested  the  common  name  "walking  fern." 
A  fourth  method  is  by  means  of  stolons  or  "runners" 
(Fig.  18). 

10.  Dispersal  of  Spores. — After  the  spores  are  mature 
the  essential  need  is  that  they  become  dispersed,  so  that 
they  may  find  favorable  conditions  of  moisture,  tem- 
perature, light,  and  soil  for  development;  for,  with  rare 
exceptions,   such  conditions   do   not  obtain  within  the 
spore-case.     Moreover,  if  the  spores  remained  within  the 
sporangia  they  would  be  so  greatly  crowded  that  only  a 
very  small  percentage  of  them  would  be  able  to  develop 
into  new  plants.     When  the  spores  are  ripe  the  spore-case 
opens,  and  by  various  movements  the  spores  are  expelled, 
often   to   a  considerable  distance;  by  wind  and  other 
agencies  they  may  be  carried  still  further  from  the  parent 
plant. 

11.  Germination  of  Spores. — After  dispersal,  and  under 
favoring  conditions  of  temperature,  moisture  and  light 
the  spore  begins  to  absorb  water,  and  soon  commences 
to  grow.     As  the  internal  pressure  increases,  the  walls  of 
the  spore  are  burst  apart,  and  a  tiny  tube,  the  germ-tube 
or   protonema    (first    thread),    begins    to   develop.     This 
process  is  germination.     Shortly,  near  the  wall  of  the  spore, 
a  smaller,  slender  tube  develops  as  a  branch  of  the  germ- 
tube  (Fig.  19).     This  is  the  first  of  innumerable  root-like 
bodies,  or  rhizoids,  which  will  help  to  hold  the  new  plant 


1 8  HEREDITY   AND   EVOLUTION   IN   PLANTS 


FIG.  19. — Germination  of  the  spores  of  a  fern,  a,  Before  germination; 
b,  early  stage,  showing  protonema  (pr.),  and  first  rhizoid  (rh}'}  c,  d,  e,  f, 
successive  stages  in  the  development  of  the  prothallus. 


FIG.  20. — Prothallus  of  a  fern.  Archegonia  on  the  (central)  cushion, 
near  the  notch;  antheridia  among  the  rhizoids,  below.  (After  Margaret 
C.  Ferguson.) 


LIFE   HISTORY   OF   A   FERN  IQ 

firmly  to  the  soil,  and  also  serve  to  take  in  water  and  dis- 
solved mineral  nutrients. 

12.  The  Prothallus. — Before  the  germ-tube  has  greatly 
enlarged,  it  becomes  divided  into  two  cells,  and  then, 
by  successive  cell-divisions,  into  an  increasing  number. 
Meanwhile  chlorophyll  bodies  begin  to  appear,  but  never 
in  the  rhizoids.  The  final  product  of  these  cell-divisions 
and  growth  is  a  tiny,  flat,  green  body,  often  (but  not 
always)  heart-shaped,  with  a  central  portion,  the  cushion, 
several  cells  thick,  and  a  marginal  part,  the  wings,  of  only 
one  cell  in  thickness.  Because  of  its  flatness  this  little 
plant  (for  such  it  is)  is  called  a  thallus;  and  because  it 
precedes,  in  the  order  of  reproduction,  the  new  sporophyte, 
it  is  called  the  prothallus  (Fig.  20).  It  is  usually  possible 
to  divide  the  prothallus  into  right  and  left  halves,  similar 
in  shape  and  in  other  characters,  and  hence  it  is  said  to 
possess  bilateral  symmetry. 


CHAPTER  II 
LIFE  HISTORY  OF  A  FERN  (Concluded) 

The  prothallus,  as  just  described,  bears  little  resem- 
blance, indeed,  to  the  fern  plant  with  which  we  are  com- 
monly familiar.  In  fact ,  the  relation  between  the  two  was 
not  understood,  nor  even  suspected,  until  about  1848, 
when  Count  Lesczyc-Suminski,  a  Polish  botanist,  first 
gave  a  connected  description  of  the  life  history  of  the  fern. 
We  shall  now  proceed  to  follow  the  steps  which  lead  from 
the  prothallus  to  the  new  sporophyte. 

13.  Dorse-ventral  Differentiation. — The  appearance  of 
the  first  root-like  body,  or  rhizoid,  was  noted  above. 
As  the  prothallus  develops  the  rhizoids  become  more  and 
more  numerous,  forming  a  mass  of  fine  thread-like  bodies 
on  the  under  side,  opposite  the  notch,  of  the  heart-shaped 
prothallus.  The  presence  of  rhizoids,  and  of  other  struc- 
tures soon  to  be  described,  makes  it  easy  to  distinguish 
at  once  the  surface  that  bears  them  from  the  opposite 
surface.  Since  the  surface  bearing  the  rhizoids  lies  nor- 
mally next  to  the  substratum  it  was  called  the  ventral 
surface,  while  the  opposite  surface  was  called  dorsal.  As 
now  used,  the  terms  dorsal  and  ventral  are  morphological 
terms,  and  have  no  reference  to  the  manner  in  which  the 
prothallus  lies.  Normally  the  ventral  surface  is  the  under 
one  and  the  dorsal  surface  the  upper,  but  the  application 
of  the  terms  would  not  be  changed  if  the  differentiated 
prothallus  should_happen,  by  any  chance,  to  lie  upside 

?o 


LIFE   HISTORY   OF   A   FERN  21 

down.  The  dorsal  surface  would  then  be  the  under 
surface,  and  the  ventral  surface  the  upper  one.  Organisms 
or  organs  having  two  such  surfaces  clearly  distinguishable 
are  said  to  have  dor  so-ventral  differentiation.  Among  many 
other  structures  thus  differentiated  are  foliage-leaves, 
sporophylls,  man,  fishes,  and  other  animals.  In  buds  the 
dorsal  surface  of  leaves  is  the  upper  or  outer  surface; 
when  foliage  leaves  are  fully  expanded  the  dorsal  surface 
is  commonly  underneath,  and  the  ventral  surface  above. 


FIG.  21. — Archegonia  of  a  fern  (Adiantum).  A,  young  archegonium; 
B,  mature;  C,  top  view,  showing  terminal  cells  of  the  four  rows  of  wall 
cells;  v,  wall  of  venter;  e,  egg;  v.c.c,  ventral  canal-cell;  n.c,  neck-canal; 
sp,  sperms  entering  the  neck-canal.  A  and  B  in  longitudinal  section. 

14.  Reproductive  Organs:  Archegonia. — Examination 
of  the  ventral  surface  of  a  mature  prothallus  with  a  lens 
will  reveal  near  the  notch  and  on  the  cushion,  several 
tiny  flask-shaped  bodies,  the  archegonia.  Each  arche- 
gonium consists  of  a  wall,  one  cell  thick,  and  contents 
(Fig.  21).  The  neck  projects  away  from  the  surface 


22  HEREDITY   AND   EVOLUTION   IN   PLANTS 

and  is  usually  slightly  curved,  while  the  remainder,  the 
venter,  is  imbedded  in  the  tissue  of  the  cushion.  As  the 
archegonium  approaches  maturity  it  is  seen  to  contain 
three  cells;  a  long  neck-canal  cell,  nearly  filling  the  neck, 
an  egg-cell  or  ovum,  filling  the  venter,  and  between  these 
two  a  ventral-canal  cell.  The  egg  is  the  female  reproduc- 
tive cell.  As  it  matures,  the  other  two  cells  become  disin- 
tegrated into  a  mucilaginous  mass  that  fills  the  neck-canal. 
Since  the  archegonia  contain  the  eggs  they  are  the  female 
reproductive  organs. 


FIG.  22. — Portion  of  a  cross-section  of  a  prothallus  of  a  fern  (Adian- 
tum),  showing  an  antheridium  (an),  and  sporogenous  cells  within. 
(Drawn  from  preparation  of  E.  W.  Olive.) 

15.  Reproductive  Organs :  Antheridia.— Search  among 
the  rhizoids  will  reveal  another  class  of  organs,  the  an- 
theridia,  globular  and  also  having  walls  only  one  cell 
thick.  These  are  the  male  reproductive  organs.  At 
maturity  they  contain  a  large  number  of  tiny  motile  cells, 
composed  chiefly  of  a  coiled  nucleus,  and  able  to  swim 
about  in  water  by  the  vigorous  lashing  of  numerous  little 
thread-like  cilia  attached  to  one  end.  These  are  the 
sperms,  or  male  reproductive  cells  (Figs.  22  and  23.) 


LIFE   HISTORY   OF    A   FESN  23 

16.  Fertilization. — Neither  the  eggs  nor  the  sperms  are 
able,  independently,  to  reproduce  their  kind.  In  order 
to  accomplish  this  they  must  unite,  and  the  fusion  of  the 
sperm  and  egg  is  fertilization.  One  of  the  most  significant 
facts  about  fertilization  in  ferns  is  that  free  water  is  re- 
quired, in  order  that  the  sperms  may  reach  the  egg  by  their 
own  locomotion.  When  the  antheridia  and  archegonia 


FIG.  23. — Fern  prothallus;  cross-sections  showing  antheridia  (an), 
sperms  (sp),  and  rhizoids  (rh).  Below  at  the  right  is  a  sperm  (sp) 
greatly  enlarged. 

are  mature,  a  suitable  amount  of  water  (such  as  would 
result  from  a  rain  or  a  copious  dew) ,  soaking  through  the 
archegonial  walls,  will  cause  the  mucilaginous  matter  in 
the  neck-canal  to  swell.  This  in  turn  will  rupture  the 
archegonia  at  their  distal  ends,  and  a  portion  of  the  con- 
tents of  the  neck-canal  will  become  extruded,  while  the 
egg  will  remain  in  the  venter.  The  same  conditions  of 


24 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


moisture  will  cause  the  rupture  of  the  antheridia,  and  the 
sperms  will  be  set  free  (Fig.  23).  The  mucilaginous  matter 
extruded  from  the  archegonia  contains  a  substance  (malic 
acid,  in  some  ferns)  which  stimulates  the  sperms  to  swim 
toward  it.  This  they  are  enabled  to  do  by  the  free 
external  water.  On  reaching  the  archegonia,  they  enter 
it,  and  swim  down  the  neck-canal  to  the  egg.  The  sperm 
that  first  reaches  the  egg  penetrates  it,  and  passes  through 


FIG.  24. — Fertilization  in  the  fern,  Onoclea.  A,  longitudinal  section 
of  archegonium,  showing  the  egg  in  the  venter,  and  numerous  sperms 
passing  down  the  neck-canal.  B,  an  egg-cell  in  the  venter.  One  sperm 
has  entered  the  nucleus,  three  sperms  have  failed  to  enter  the  egg.  (After 
W.  R.  Shaw.) 

its  cytoplasm  until  it  reaches  the  egg-nucleus,  with  which 
it  fuses,  thus  completing  the  act  of  fertilization  (Fig.  24). 
As  soon  as  one  sperm  enters  the  egg-cell,  the  latter  at  once 
forms  a  fertilization-membrane  about  itself,  through  which 
the  remaining  sperms  cannot  enter. 


LIFE   HISTORY   OF    A  FERN 


17.  Nature  of  the  Fertilized  Egg.— It  will  at  once  be 
recognized  that  the  fertilized  egg,  resulting  from  a  union 
with  the  sperm,  possesses  a  double  or  diploid  nature.1 
In  recognition  of  its  dual  nature  it  is  called  the  oosperm 
(egg  and  sperm).2  The  6'osperm,  however,  like  the  un- 


FIG.  25. — Young  embryo  of  a  maidenhair  fern  (Adiantum  concinnum), 
still  surrounded  by  the  archegonium,  which  has  grown  in  size.  L,  leaf; 
S,  stem;  R,  root;  F,  foot.  (After  Atkinson.) 

fertilized  egg,  is  still  only  one  cell,  though  its  nucleus  com- 
prises substances  contributed  by  both  egg  and  sperm. 
In  some  cases  the  egg  and  sperm  that  unite  in  fertilization 
may  come  from  different  parents;  their  fusion  is  then 
called  cross-fertilization. 

1  As  distinguished  from  the  unfertilized  egg,  which  is  of  a  single,  or 
haploid  nature. 

2  The  term  oospore  is  often  used  here,  but  this  term  lacks  the  advan- 
tage of  indicating  the  real  nature  of  the  fertilized  egg. 


26  HEREDITY   AND   EVOLUTION  IN   PLANTS 

18.  Development  of  the  Fertilized  Egg. — After  fertili- 
zation the  egg  begins  to  develop,  undergoing  a  series  of 
nuclear  and  cell-divisions,   accompanied  by  increase  in 
size.     The  cell-wall  of  the  first  division  (in  all  of  the  family 
Polypodiaceae)  is  parallel  to  the  axis  of  the  archegonial 
neck.     The  second  wall,  at  right  angles  to  the  first,  di- 
vides the  oosperm  into  four  cells.     The  beginning  of  these 
divisions  marks  the  beginning  of  the  embryo.     By  further 
cell-divisions  each  of  the  first  four  cells  develops  a  mass  of 
embryonic  tissue.     The  two  cells  on  one  side  of  the  first 
wall  formed  represent,  the  one  the  embryonic  stem,  and 
the  other  the  embryonic  leaf,  or  cotyledon.     One  of  the 
two  cells  on  the  opposite  side  of  the  first  wall,  develops 
into  the  embryonic  root,  while  the  other  develops  into  an 
organ  peculiar  to  the  embryonic  stage,  and  known  as  the 
foot  (Fig.  25).     The  function  of  the  foot  is  to  absorb 
nourishment  for  the  young  embryo  from  the  prothallus. 
The  need  of  such  an  organ  becomes  apparent  when  it  is 
recalled  that  the  oosperm,  and  consequently  the  embryo, 
lie  free  in  the  venter  of  the  archegonium,  without  any 
organic  or  structural  connection   with    the  prothallus. 
This  necessary  connection  is  early  established  by  the  foot. 

19.  Growth  of  the  Embryo. — As  the  embryo  continues 
to  grow,  the  root  develops  first.     The  advantage  of  this 
will  become  evident  when  we  remember  that  the  primary 
and  most  fundamental  need  of  the  young  plant  is  water, 
which  is  taken  in  by  the  roots.     The  next  most  funda- 
mental need  is  nourishment,  and  as  plant  food  is  manufac- 
tured   in    chlorophyll-bearing    organs,    and    usually    in 
leaves,  we  would  expect  the  early  development  of  leaves. 
Such  is  the  case,  the  growth  of  the  first  leaf  being  second- 
ary only  to  that  of  the  root,  and  in  advance  of  the  stem. 


LIFE    HISTORY    OF    A   FERN  27 

The  development  of  the  stem  follows,  and  finally  spore- 
bearing  leaves  appear  (Fig.  26).  We  then  have  an 
organism  similar  to  that  with  which  we  started — a  full- 
grown  fern-plant,  capable  of  producing  spores,  which  can 
develop  into  prothallia  again,  with  antheridia  and  arche- 
gonia,  producing  sperms  and  eggs,  and  so  on.  Thus  we 
see  that  the  steps  in  the  life  history  of  a  fern  constitute 
a  life-cycle.  At  whatever  point  or  with  whatever  struc- 


FIG.  a6. — Prothallia  of  a  fern,  i,  Before  the  sporophyte  had  appeared; 
2-5,  with  sporophytes  attached;  /,  cotyledon  or  first  leaf  of  the  sporophyte; 
v,  circinate  vernation  of  a  leaf;  s,  mass  of  soil. 

ture  we  start,  if  we  follow  the  course  of  development  we 
are  brought  back  again  to  the  same  point,  or  the  same 
kind  of  structure  with  which  we  began. 

20.  Simpler  Ferns. — In  addition  to  the  leptosporan- 
giate  ferns,  which  have  served  as  a  basis  for  the  general- 
ized description  given  above,  there  is  another  group, 
having  a  more  primitive  type  of  organization.  Repre- 
sentatives of  this  group  include  the  "moonworts"  (species 
of  Botrychium,  Fig.  27),  and  the  "adder's  tongue"  (Oph- 


28  HEREDITY   AND   EVOLUTION   IN   PLANTS 

ioglossum   vulgatum,    Fig.    28).     The    species    of   Botry- 
chium  usually    (though  not  invariably)  possess  but  one 


FIG.  27. — Rattlesnake  fern  (Botrychium  nrginianum  (L.)  Svv.). 

foliage-leaf,  and  a  fertile  spike,  both  of  which  are  more  or 
less  branched.  Abnormal  forms  are  not  uncommon  in 
which  the  fertile  spike  is  more  or  less  sterilized,  sometimes 


LIFE   HISTORY   OF   A  FERN 


being  entirely  so;  while  in  other  cases  sporangia  occur  on 
the  foliage-leaf.  As  in  the  re- 
placement of  sporophylls  by 
sterile  leaves  in  the  ostrich  fern, 
Onoclea  struthiopteris  (para- 
graph 8),  these  abnormalities 
indicate  the  close  relationship 
between  leaves  and  spore-bear- 
ing organs,  and  clearly  show 
that  the  latter  may  be  com- 
pletely transformed,  by  sterili- 
zation, into  foliage-leaves. 

In  Ophioglossum  the  foliage- 
leaf  and  spore-bearing  spike 
are  both  unbranched,  the  latter 
suggesting  an  adder's  tongue, 
whence  the  name,  Ophioglossum. 
In  both  Ophioglossum  and  Botry- 
chium  the  sporangia  originate 
from  a  group  of  epidermal  and 
sub-epidermal  cells,  and  are 
consequently  imbedded  in  the 
surrounding  tissue.  Their  walls 
are  more  than  one  cell  in  thick- 
ness, the  annulus  is  lacking,  and 
they  open  by  a  slit.  Ferns  of 
this  type  are  called  eusporangiate 
(Cf.p.io).  Their  pro  thallia  are 
usually  fleshy  and  subterranean, 
bear  the  antheridia  and  arche- 
gonia  on  the  dorsal  instead  of  on 
the  ventral  surface,  and  are  per- 


FIG.  28. — Adder's  tongue 
fern  (Ophioglossum  •ovlgatum 
L.).  R,  runner  or  stolon. 


30  HEREDITY  AND   EVOLUTION  IN   PLANTS 

ennial,  often  living  on  after  the  sporophyte  has  died.  In 
general  the  sporophyte  possesses  less  sterile  tissue  in  pro- 
portion to  fertile  tissue  than  is  the  case  with  the  lepto- 
sporangiate  forms.  These  characters  mark  the  group  as 
more  primitive  than  the  leptosporangiate  ferns,  and  they 
are  much  less  numerous,  only  about  100  species  being 
known  from  the  entire  world,  while  of  the  leptosporangiate 
ferns  between  3,000  and  4,000  species  have  been  described. 
Recent  studies  of  the  vascular  anatomy  of  the  Ophio- 
glossacese  have  disclosed  features  in  common  with  the 
Osmundaceae  and  Polypodiaceae.  The  fact  that  the  vas- 
cular bundles  of  the  fertile  spike  originate  in  the  same 
manner  as  those  extending  into  the  pairs  of  pinnae  of 
the  sterile  segment  points  to  the  conclusion  that  the  fertile 
spike  represents,  or  is  homologous  with,  two  fused  pinnae 
at  the  base  of  a  fern  leaf.  From  this  and  other  evidence 
the  Ophioglossaceae,  while  "simpler"  in  structural  fea- 
tures, have  been  regarded  as  not  having  had  a  strobilar 
origin  (by  progressive  sterilization1)  from  the  liverworts, 
and  as  not  standing  in  the  ancestral  line  of  the  modern  lep- 
tosporangiate ferns,  but  as  having  themselves  been  derived 
at  a  very  early  period  from  a  primitive  fern  stock  closely 
related  to  the  Osmundaceae.  On  the  other  hand,  Camp- 
bell2 has  adduced  evidence  for  the  derivation  of  the  fertile 
spike  of  Ophioglossum  from  a  sporogonium  like  that  of  the 
liverwort,  Anthoceros.  This  and  other  evidence  indicates 
that  the  Ophioglossaceae,  and  the  eusporangiate  ferns 
as  a  group,  are  the  oldest  fern  stock,  and  this  conclusion 
is  supported  by  the  geological  record,  for  the  oldest  known 
fossil  ferns  are  eusporangiate.  Further  investigation  is 
necessary  before  the  question  can  be  definitely  settled. 

JCf.  pp.  379,  432,  and  574  infra. 

2  Campbell,  D.  H.,  Amer.  Nat.  41 :  139-159.     1907. 


CHAPTER  III 
FUNDAMENTAL  PRINCIPLES 

21.  Two  Kinds  of  Reproduction. — In  the  two  preced- 
ing chapters  attention  has  been  called  to  three  ways  of  ob- 
taining new  fern-plants,  namely,  by  spores,  by  vegetative 
multiplication,  and  by  fertilized  eggs.  The  first  two 
methods  may  be  grouped  together  as  asexual,  while  the 
second  is  sexual,  as  shown  in  the  following  table. 


By  the  giving  off  of 

Artificial    (slips, 

multi-cellular    por- 

cuttings, etc.). 

Asexual,     in- 

tions or  outgrowths 

Natural  (tubers, 

volving  cell- 

of  vegetative  tissue. 

bulbs,  gemmae)  . 

divisions 

By  the  giving  off  of 

only. 

special  reproductive 

bodies  of  one  or  few 

cells,  called  spores. 

Sexual,      in- 

volving cell- 

fusions. 

Reproduction 


22.  Vegetative  Multiplication. — Vegetative  multipli- 
cation may  be  accomplished  either  without  or  with  the 
intervention  of  man.  In  the  first  case  the  plant  produces 
special  reproductive  bodies  such  as  tubers,  bulbs,  offsets, 
and  stolons,  which  become  separated  from  the  plant  with- 
out assistance,  and  develop  into  new  individuals.  In 
the  second  case  a  similar  result  is  accomplished  through 
the  removal  by  the  gardener  of  portions  of  the  parent ' 
plant,  such  as  slips,  cuttings,  leaves  (e.g.,  in  the  begonia), 
or  by  bending  branches  over  until  they  touch  the  ground, 
and  there  take  root,  after  which  the  newly  rooted  portion 
31 


32  HEREDITY   AND   EVOLUTION   IN  PLANTS 

may  be  severed  from  the  parent  plant.  This  is  called 
layering.  The  production  of  new  individuals  by  the  arti- 
ficial methods  of  the  gardener  is  called  propagation;  but 
between  these  methods  and  multiplication  by  special 
bodies,  given  off  spontaneously  by  the  plant,  no  hard  and 
fast  line  can  be  drawn.  Some  plants,  for  example,  be- 
come layered  without  the  gardener's  assistance;  other 
plants  (as  the  willow),  by  self -pruning,  spontaneously 
give  off  branches  from  which  new  plants  may  develop; 
while,  on  the  other  hand,  the  gardener  may  cut  a  tuber, 
such  as  the  "potato"  into  a  number  of  pieces,  from  each  of 
which  a  new  plant  will  develop.  In  this  practice  artificial 
propagation  and  vegetative  multiplication  are  combined. 

23.  Reproduction  by  Spores. — The  essential  fact  about 
a  spore  is  that  it  is  an  individual  cell  or  small  group  of 
cells,    produced    primarily    for    reproductive    purposes, 
given  off  by  the  plant,  and  capable  by  itself  of  producing 
a  new  individual.     The  essence  of  all  reproduction  is  the 
separation  of  the  reproducing  cell  or  body  from  the  parent 
plant.     If  a  bud  or  a  bulb  remains  attached  to  the  plant 
that  formed  it,  it  produces  only  a  branch  or  other  organ, 
but  not  a  new  individual.     So,  also,  a  spore  must  be  sepa- 
rated from  the  parent  plant  in  order  to  reproduce  the 
latter.     In  many  cases  spores  may  germinate  before  they 
are  set  free,  but  the  separation  must  come  sooner  or  later. 

24.  Sexual    Reproduction. — In    marked    contrast    to 
reproduction  by  spores,  is  the  reproduction  by  means  of 
sperms  and  eggs,  invoking  cell-  and  nuclear-fusions,  known 
as  fertilization.     Eggs  and  sperms  are  called  gametes,1 
the  egg  being  the  female  gamete,   the  sperm  the  male 
gamete.     The  diploid  cell,  resulting  from  the  union  of  two 
gametes,  is  called  a  zygote,  and  this  term  is  often  extended 

1  From  the  Greek  word,  7^0$  (games),  meaning  marriage. 


FUNDAMENTAL   PRINCIPLES  33 

to  apply  to  the  resulting  diploid  organism  through  all 
stages  of  its  development  to  maturity. 

25.  Two  Kinds  of  Generations. — A  study  of  the  life 
history  of  the  fern  disclosed  two  distinct  phases  or  genera- 
tions, one  bearing  spores,  and  therefore  called  the  sporo- 
phyte  (spore-bearing  plant),  the  other  bearing  gametes  and 
for  that  reason  called  the  gametophyte  (gamete-bearing 
plant).  The  gametophyte  of  the  fern  was  seen  to  be 
entirely  independent  of  the  sporophyte,  capable  of  manu- 
facturing its  own  food  by  means  of  its  own  chlorophyll, 
not  dependent  upon  any  other  plant,  and  in  some  groups 
being  perennial,  living  on  from  year  to  year,  and  giving 
rise  to  sporophytes  that  live  for  only  one  season.  The 
sporophyte,  on  the  other  hand,  is  at  first,  entirely  de- 
pendent upon  the  gametophyte  for  its  nutrition,  living  as 
a  parasite  upon  the  prothallus,  from  which  it  absorbs  its 
nourishment  by  means  of  the  special  organ,  the  foot. 
Gradually,  however,  the  sporophyte  puts  forth  roots, 
capable  of  taking  in  water  and  dissolved  mineral  sub- 
stances from  the  soil,  and  chlorophyll-bearing  organs  the 
fronds  or  leaves),  capable  of  manufacturing  organic  food. 
As  the  sporophyte  becomes  independent,  the  gameto- 
phyte (with  few  exceptions,  as  noted  above),  perishes. 
A  comparison  of  the  two  generations  shows  that  the 
sporophyte  is  the  much  more  complex  of  the  two,  being 
clearly  differentiated  into  roots,  and  leafy  shoot.  The 
difference  in  the  origin  of  these  two  generations  results  in 
a  very  fundamental  difference  in  the  nature  of  all  the 
cells  in  each.  Since  the  sporophyte  is  derived  from  an 
oosperm  (zygote),  formed  by  the  fusion  of  the  two, 
gametes,  all  of  its  cells  are  diploid,  containing  material 
derived  from  both  its  male  and  female  parentage.  The 


34  HEREDITY   AND   EVOLUTION   IN   PLANTS 

gametophyte,  on  the  other  hand,  being  derived  from  a  sin- 
gle reproductive  cell  (the  spore),  without  nuclear  or  cell-fu- 
sions, is  composed  of  cells  of  a  single  or  haploid  nature. 

26.  Alternation  of  Generations.— Our  study  of  the 
fern  also  brought  out  another  fact  of  very  fundamental 
importance.  Sporophytes  do  not  produce  sporophytes, 
nor  gametophytes,  gametophytes ;  but  there  is  always 
an  alternation  of  generations,  sporophytes  producing 
gametophytes,  and  gametophytes,  sporophytes. 

The  order  of  sequence  in  the  life-cycle  is  as  follows: 

sporophyte — »spore — ^gametophyte — >gametes — H)6sperm — ^sporophyte. 

The  order  of  structures  and  processes  involved  in  the 
life-cycle  is  as  follows: 

OUTLINE  OF  LIFE  HISTORY  OF  A  FERN 

Gametophyte  (prothallus) 


*  * 

Antheridium  Archegonium 

4  4 

Sperm  (male  gamete)  Egg  (female  gamete) 

Fertilization  ^^-^* 


Oosperm  (zygote) 
Embryo 

44 

Mature  sporophyte  (mature  zygote) 
Sporophyll 

44 

Sporangium 
Spore-mother-cell 


MM 

Spore  Spore  Spore    Spore 
Gametophyte 


Reduction 


FUNDAMENTAL    PRINCIPLES 


The  fact  of  a  cycle  in  the  life  history  is  brought  out 
clearly  in  the  following  diagram : 


FIG.  29. — Diagram  of  life-cycle  of  a  fern. 

27.  Reduction. — Since  the  sporophyte  (descended  from 
the  diploid  oosperm)  has  cells  of  a  double  nature,  resulting 
from  fertilization,  and  since  the  spores  which  give  rise  -to 
the  gametophyte  are  of  a  single  (or  haploid)  nature, 
there  must  occur,  at  some  stage  in  the  life  of  the  sporo- 
phyte, a  process  of  reduction,  restoring  the  cells,  made 
diploid  by  fertilization,  to  the  haploid  condition.  Pains- 
taking studies  of  cellular  structure  and  processes  has 
disclosed  the  fact  that  this  reduction  takes  place  during 
the  two  successive  divisions  (tetrad-divisions)  of  the  spore- 
mother-cell,  resulting  in  the  formation  of  four  spores. 
The  diploid  condition  persists  in  all  the  cells  of  the 
sporophyte,  and  through  every  cell-division,  up  to  the  two 
divisions  preceding  spore-formation,  just  as  the  single  or 
haploid  condition  persists  in  all  the  cells  of  the  gameto- 
phyte, up  to  the  very  act  of  fertilization. 


30  HEREDITY   AND   EVOLUTION   IN   PLANTS 

28.  Nature  and  Method  of  Reduction.— In  order 
thoroughly  to  understand  fertilization  and  reduction  one 
must  have  a  knowledge  of  the  structure  and  behavior  of 
the  nucleus  in  cell-division  and  cell-fusion.  This  subject 
is  too  difficult  and  too  extended  to  be  thoroughly  treated 


FIG.  30. — Diagram  illustrating  various  stages  of  indirect  nuclear 
division  (mitosis).  A,  resting  nucleus  of  the  mother-cell;  B,  formation 
of  nuclear  skein  or  spirem;  C,  longitudinal  splitting  of  thespirem;  D,  the 
chromosomes  (four  in  number)  have  been  formed  by  the  transverse  seg- 
mentation of  the  spirem;  E,  chromosomes  arranged  on  the  equator  of  the 
nuclear  spindle;  F  and  G,  early  and  late  anaphase,  the  chromosomes  moving 
to  the  pales  of  the  spindle;  H,  formation  of  daughter -spirems;  7,  resting 
stage  of  the  two  daughter-cells. 

in  an  introductory  study,  but  the  salient  facts  are  as 
follows.  The  nucleus  of  all  cells  comprises  at  least  four 
substances:  nuclear  sap,  a  threadwork  of  limn,  and  a 
substance  called  chromatin;1  all  these  are  enclosed  by  a 
nuclear  membrane.  In  the  non-dividing  nucleus  the 

1  Because  it  stains  readily  when  treated  with  certain  aniline  dyes. 


FUNDAMENTAL   PRINCIPLES 


37 


chromatin  is  distributed  on  the  linin  threads  in  the 
form  of  minute  granules  (Fig.  30.)  At  one  of  the  stages 
preliminary  to  nuclear  division  the  linin  network,  with  the 
chromatin,  becomes  transformed  into  a  thickened  skein, 


FIG.  31. — Diagram  illustrating  various  stages  in  the  reduction  division 
(maiosis)  of  a  spore-mother-cell  of  a  plant;  A,  resting  stage  of  the  mother- 
cell-nucleus;  B,  the  nuclear  skein  or  spirem,  in  synizesis  (during  synapsis); 
C,  the  spirem  after  synapsis,  showing  its  double  (diploid)  nature;  the  dot- 
ted line  indicates  the  segmentation  of  the  spirem  into  two  diploid  chromo- 
somes, each  of  which  has  split  longitudinally  in  D;  E,  the  diploid  chromo- 
somes on  the  equator  of  the  spindle  of  the  first  (heterotypic)  division; 
F,  late  anaphase;  G,  metaphase  of  the  second  or  homotypic  division;  H, 
late  anaphase  of  same,  two  haploid  chromosomes  approaching  the  poles 
of  each  spindle;  7,  the  four  daughter-cells  (spores)  of  the  tetrad. 

which  shortly  becomes  split  into  two,  throughout  its  entire 
length.  The  skein  finally  becomes  divided  transversely 
into  a  number  of  double  chromatin  bodies  or  chromosomes. 
The  number  of  these  chromosomes  is  characteristic,  and 
always  the  same  for  each  species  of  plant.  The  nuclear 


30  HEREDITY  AND   EVOLUTION   IN   PLANTS 

membrane  then  disappears,  and,  by  a  complicated  mechan- 
ism, not  entirely  understood,  the  two  halves  of  the  chro- 
mosomes are  separated  and  carried  apart  to  opposite  sides 


of  the  cell.  After  this  division  of  the  nucleus,  a  new  cell- 
wall  forms,  dividing  the  entire  cell  into  halves;  new  nu- 
clear membranes  develop,  and  the  chromosomes  in  each 


FUNDAMENTAL   PRINCIPLES  39 

daughter -nucleus  becomes  gradually  retransformed  into  a 
resting  nucleus,  like  the  one  with  which  we  started. 

In  reduction  (Fig.  31)  a  new  resting  nucleus  is  not 
organized  after  the  first  nuclear  division  by  which  the 
number  of  chromosomes  in  each  nucleus  is  reduced  by 
one-half,  but  this  division  is  followed  at  once  by  a  second. 
This  is  the  process  of  tetrad-division,  by  which  four 
spores  are  formed  from  each  spore-mother-cell.  The 
reduced  number  of  chromosomes  persists  throughout 
the  gametophyte-phase,  including  the  formation  of  both 
egg  and  sperm.  When  the  latter  unite,  the  nucleus  of  the 
zygote  will,  of  course,  possess  the  doubled  number  of 
chromosomes,  which  then  persists  throughout  the  body  of 
the  sporophyte  (mature  zygote),  until  the  stage  of  spore- 
formation  is  again  reached.  These  facts  are  shown  dia- 
grammatically  in  Fig.  32. 

29.  Inheritance. — It  is,  of  course,  common  knowledge 
that  men  do  not  gather  grapes  of  thorns,  nor  figs  of 
thistles.  A  given  species  of  fern  always  reproduces  the 
same  species,  and  this  is  true  of  all  plants.  It  requires 
only  a  brief  reflection  to  realize  that  this  must  be  so,  for 
the  beginning  of  every  living  thing  is  always  merely  a 
piece  of  an  antecedent  organism,  the  parent.  The  off- 
spring would,  therefore,  naturally  partake  of  the  nature  of 
its  parent — it  is  a  piece  of  it- — was  originally  a  part  of 
it.  Resemblance  between  ancestor  and  descendant  is 
commonly  regarded  as  inheritance,  but  only  a  little 
careful  thinking  will  lead  us  to  see  that  resemblance 
and  inheritance  are  by  no  means  synonymous.  The  real1 
nature  of  inheritance  is  well  illustrated  by  the  inheritance 
of  property  by  a  son  from  his  father.  The  thing  inherited 
is  not  an  external  appearance,  but  a  material  substance 


40  HEREDITY  AND   EVOLUTION   IN   PLANTS 

(land,  buildings,  a  business),  which  is  handed  from  one 
to  another.  So  it  is  in  reproduction.  That  which  one 
generation  of  plants  inherits  from  another  is  the  substance 
of  the  reproductive  cells — the  protoplasm  of  the  spore, 
oosperm,  tuber,  or  bulb — plus  a  certain  characteristic 
organization  of  this  protoplasm,  and  the  effects  of  its  past 
history. 

30.  Inheritance  Versus  Expression. — That  inherit- 
ance and  expression  are  not  the  same  thing  is  plainly 
shown  in  the  life  history  of  the  fern,  for  the  gametophyte 
clearly  derives  its  living  matter  by  inheritance  from  the 
sporophyte,  and  the  sporophyte,  in  turn,  its  living  matter 
from  the  gametophyte,  and  yet  the  two  generations  look  so 
little  alike  that  men  for  centuries  knew  them  both  with- 
out recognizing  the  fact  that  they  were  merely  two  dif- 
ferent phases  in  the  life  history  of  the  same  species  of 
plant.  So,  often,  among  human  beings,  children  may 
bear  very  little  if  any  resemblance  to  their  parents,  but 
may  closely  resemble  their  grandparents.  Clearly  we 
do  not  inherit  the  color  of  our  eyes  or  hair,  the  shapes  of 
our  noses,  the  peculiarities  of  our  voices,  or  our  mental 
traits  from  our  parents,  nor  even  from  our  more  remote 
ancestors.  What  we  do  inherit  is  a  tiny  particle  of  proto- 
plasm having  a  certain  characteristic  composition,  struc- 
ture, and  past  history.  This  protoplasm  is  capable,  under 
certain  combinations  of  circumstances,  of  developing 
into  a  mature  organism,  resembling  the  one  from  which 
it  came,  but  under  other  combinations  of  circumstances 
the  external  appearance — the  expression — may  resemble 
that  of  the  parent  only  a  very  little,  or  not  at  all.  In- 
heritance may  therefore  be  defined  as  the  recurrence  in 
successive  generations,  of  a  similar  cellular  constitution^ 

1  Following  Johannsen,  Cf.  p.  67. 


FUNDAMENTAL   PRINCIPLES  41 

Expression  of  this  cellular  condition  is  greatly  modified 
by  circumstances,  which  are  never  precisely  the  same 
for  any  two  individuals  (Cf.  p.  48) . 

31.  Variation. — The   preceding  sentence  explains,  in 
part,  why  it  is  that  no  two  individuals  are  ever  precisely 
alike — precisely   similar    circumstances    surrounding    de- 
veloping organisms  never  occur  twice;  that  is,  the  environ- 
ment varies.     Besides  this,  internal  changes  may  take 
place  in  the  reproductive  cells.     For  either  one  or  both  of 
these  reasons,  constant  variation  is  the  rule  for  living 
things.     This  subject  will  be  considered  more  at  length 
in  Chapters  V  and  VI. 

32.  Adjustment    to    Environment. — By    the   term 
environment  is  meant  all  the  circumstances  that  surround 
a  cell,  tissue,  or  organism  at  any  given  time,  or  throughout 
its  existence.     The  environment  of  tissues  and  organs 
includes  surrounding  tissues  and  organs,  and  the  environ- 
ment of  cells  includes  the  neighboring  tissues  and  cells. 
The  most  essential  thing  in  the  life  of  every  plant  or  animal 
is  to  keep  in  harmony  with  its  environment.     Every  change 
of  environment  necessitates  an  adjustment  on  the  part 
of  the  plant  in  order  to  maintain  this  harmony.     Adjust- 
ments are  most  easily  made  when  the  plant  is  young  and 
plastic,  and  especially  while  it  is  developing  to  maturity. 
If  the  amount  of  water  in  the  soil  is  diminished  the  young 
plant  will  send  its  roots  deeper,  if  light  is  entirely  cut  off  no 
chlorophyll  will  form.     A  leaf,  or  the  prothallus  of  ferns,  is 
bilaterally  symmetrical  partly  because  the  environment  is 
uniform  on  all  sides;  the  same  organs  have  dorso-ventral , 
differentiation  largely  because  the  environment  is  unlike 
above  and  below.     The  motility  of  sperms  is  an  adjustment 
to  water  in  the  environment.     Thus,  variations  in  the 


42  HEREDITY   AND   EVOLUTION   IN   PLANTS 

environment  may  result  in  different  expressions  of  in- 
heritance, just  as  variations  in  inheritance  would  be 
followed  by  differences  in  expression,  even  in  an  unchang- 
ing environment.  In  order  correctly  to  understand  a 
plant  nothing  is  more  necessary  than  to  remember  that 
its  characteristics  are  the  result,  not  of  its  inheritance 
alone,  nor  of  its  environment  only,  but  of  the  interaction 
between  the  two. 

33.  Struggle  for  Existence. — In  paragraph  7  atten- 
tion was  called  to  the  fact  that  a  moderate-sized  fern  pro- 
duces each  year  about  50,000,000  spores.  If  each  one  of 
these  spores  ultimately  produced  a  mature  fern-plant,  and 
if  we  allowed  only  i  square  foot  of  "elbow-room"  for  each 
plant,  the  progeny  of  one  parent  only,  in  one  season 
would  require  at  least  50,000,000  square  feet,  or  nearly  i% 
square  miles.  If  each  of  these  plants  in  turn,  produced 
50,000,000  offspring  the  next  season,  the  descendants  of 
only  one  fern  plant  would,  in  only  two  years,  cover  the 
stupendous  area  of  over  83,000,000  square  miles,  or  an 
area  equal  to  that  of  the  North  American  Continent. 
It  has  been  calculated  that  a  single  plant  of  hedge  mustard 
may  produce  as  many  as  730,000  seeds.  If  each  seed 
developed  another  full-grown  plant,  and  if  the  plants  were 
distributed  73  to  each  square  meter,  there  would  be  enough 
mustard  plants  to  cover  an  area  equal  to  2,000  times 
the  dry  surface  of  the  earth.  One  may  easily  imagine 
the  result  if  all  the  seeds  produced  by  one  of  our  large 
forest  trees  were  able  to  mature.  And  yet  the  total 
number  of  any  given  kind  of  fern,  of  hedge  mustard,  or 
of  forest  tree  does  not  appreciably  change  from  year  to 
year.  The  reason,  of  course,  is  that  not  all  of  the  spores 
and  seeds  produced  are  allowed  to  come  to  maturity. 


FUNDAMENTAL   PRINCIPLES  43 

The  direct  result  of  the  enormous  number  of  spores  and 
seeds  produced  is  a  struggle  for  existence — for  sufficient  soil, 
water,  light,  and  food  to  insure  a  healthy,  mature  plant. 

34.  Elimination  of  the  Unfit. — As  a  result  of  variation 
certain  individuals  will  succeed  better  than  others  in  the 
struggle  for  existence.     Those  most  poorly  adapted  to 
their  surroundings  will  perish,  and  only  the  more  vigorous 
ones— those   best   adjusted   to   their   surroundings — will 
persist.     The  result  of  this  struggle  for  existence  was 
called  by  Herbert  Spencer  the  "survival  of  the  fittest" 
What  really  takes  place  in  nature  is  the  elimination,  by 
death,  of  the  unfit.     Darwin  called  this  natural  selection, 
implying  that  the  result  is  similar  to  that  when  plant 
breeders  select  out  of  a  progeny  the  best  individual  for 
further  breeding.     What  really  takes  place  in  nature, 
however,  is  not  so  much  the  selection  of  the  fittest,  but  a 
rejection    of   the   unfit.     Thus,    among    the    50,000,000 
progeny  of  a  single  fern-plant,  some  are  sure  to  have  a 
weaker  constitution  than  others;  to  develop  a  weaker  root- 
system,  less  chlorophyll  in  their  leaves,   a  less  number 
of  sporophylls  or  spores,  or  to  be  inferior  in  other  ways. 
The  result  will  be  that,  in  the  course  of  only  a  few  years, 
the  descendants  of  the  most  vigorous  or  otherwise  superior 
plants  will  alone  be  left  to  perpetuate  the  race. 

35.  Problems  to  Solve. — In  the  preceding  paragraphs 
we  have  called  attention  to  a  number  of  the  problems 
which  arise  from  the  study  of  a  fern.     Some  of  these  have 
been  partially  solved — probably  none  of  them  has  been 
completely  solved.     In  fact,  we  may  say  that  our  igno-' 
ranee  of  life-processes  greatly  exceeds  our  knowledge. 
Very  much  more  remains  to  be  ascertained  than  has  al- 
ready been  found  out;  for  example,  what  is  protoplasm? 


44  HEREDITY   AND   EVOLUTION   IN   PLANTS 

Nobody  really  knows.  We  have  analyzed  the  substance 
chemically,  we  have  carefully  examined  and  tried  (but 
without  complete  success)  to  describe  its  structure.  We 
know  it  is  more  than  merely  a  chemical  compound.  It 
is  a  historical  substance.  A  watch,  as  such,  is  not.  The 
metal  and  parts  of  which  a  watch  is  made,  have,  it  is  true, 
a  past  history;  but  the  watch  comes  from  the  hands  of  its 
maker  de  novo.  without  any  past  history  as  a  watch. 
But  not  so  the  plant  cell.  It  has  an  ancestry  as  a  cell; 
its  protoplasm  has  what  we  may  call  a  physiological  mem- 
ory of  the  past.  It  is  what  it  is,  not  merely  because  of  its 
present  condition,  but  because  its  ancestral  cells  have  had 
certain  experiences.  We  can  never  understand  a  plant 
protoplast  by  studying  merely  it ;  we  must  know  something 
of  its  genealogy  and  its  past  history. 

What  is  the  origin  of  the  sporophyte,  and  how  did  there 
come  to  be  two  alternating  generations?  What  is  the 
meaning  of  fertilization;  what  the  mechanism  and  laws 
of  inheritance?  How  did  there  come  to  be  on  the  earth 
such  plants  as  ferns ?  What  was  the  origin  of  life?  What 
is  life?  No  one  can  give  complete  answers  to  these  ques- 
tions; but  the  purpose  of  the  study  of  botany  is  to  help 
fit  us  to  seek  the  answers  intelligently.  To  those  who  are 
interested  in  problems  of  this  sort,  nothing  can  be  more 
fascinating,  nor  more  profitable.  It  .is  the  aim  of  the  fol- 
lowing chapters  to  give  a  brief,  elementary  resume  of  the 
method  employed  and  the  results  obtained  during  the 
past  fifty  years  by  investigators  in  their  attempts  to  solve 
two  of  the  more  important  of  these  problems,  namely, 
the  nature  and  mechanism  of  inheritance  and  the  causes 
and  course  of  plant  evolution. 


CHAPTER  IV 
HEREDITY 

36.  Importance  of  the  Study. — i.  To  Pure  Science. — 
No  knowledge  is  more  fundamental  than  a  correct  under- 
standing of  the  aws  of  heredity.  Its  fundamental  im- 
portance to  pure  science  becomes  evident  at  once  when  we 
consider  that,  since  evolution  has  been  accomplished  by 
the  descent  of  one  organism  from  another,  there  have  been 
one  or  more  unbroken  lines  of  inheritance  from  the  dawn 
of  plant  life  to  the  present.  Hence,  until  we  know  the 
laws  of  heredity,  we  cannot  fully  understand  expression, 
reproduction,  development,  variation,  sex,  or  evolution. 

2.  To  Applied  Science. — Correct  ideas  concerning  he- 
redity are  absolutely  essential  to  such  phases  of  applied 
science  as  animal  and  plant  breeding.     In  the  light  of  such 
knowledge  the  breeder  can  avoid  making  useless  experi- 
ments, and  can  accomplish  desired  results  more  quickly, 
more  cheaply,  and  with  greater  certainty  of  success. 

3.  To  Man. — A  correct  knowledge  of  the  principles  of 
heredity  is  vital  to  mankind;  no  knowledge  is  more  so.     To 
realize  this,  we  have  only  to  reflect  that  our  own  characters 
are  very  largely  the  result  of  inheritance  from  our  ances- 
tors; and  not  only  our  characters,  but  our  physical  char- 
acteristics, our  vigor  of  m'nd  and  body,  our  capacity  for 
education,  our  susceptibility  to  disease,  and  often  the 
actual  existence  of  some  disease  within  our  bodies  or  minds. 

45 


46 


HEREDITY  AND    EVOLUTION   IN  PLANTS 


37.  Heredity  Reduced  to  Its  Lowest  Terms.—  We  may 

study  heredity  under  the  very  simplest  conditions  in  the 
descent  of  one-celled  organisms,  such  as  Pleurococcus. 
This  plant,  a  unicellular  green  alga,  is  a  globule  of  proto- 
plasm, containing  chlorophyll,  and  surrounded  by  a 
cellulose  cell-  wall  (Fig.  33).  But  why  is  it  globular,  why 
does  it  contain  chlorophyll,  why  has 
it  a  cell-  wall  of  cellulose?  Why  is 
it  not  elliptical,  why  is  it  not  red  in- 
stead of  green,  why  does  it  have  a 
cell-wall,  instead  of  existing  naked 
like  the  plasmodium  of  a  slime- 
mold,  why  is  its  cell-wall  of  cellulose, 
rather  than  of  lignin  or  chitin? 

The  short  answer  is,  because  its 
ancestors,  for  ages  and  ages,  have 
P™  d  the  characteristics  which 
(Pleurococcus  vulgar  is)  now  characterize  Pleurococcus 
showing  the  tendency  of  plants.  But  that  only  puts  the 
the  cells  to  remain  question  back  an  indefinite  number 

attached    after    cell-dw-    n 

sion,  thus  causing  transi-  of  generations.  The  real  reason  is, 
tions  from  a  one-celled  to  because  ihePleurococcus  protoplasm 
a  multi-cellular  plant,  possesses  a  physical  and  chemical 
constitution  —  or  in  other  words  a 
mechanism  —  that,  under  normal  external  conditions, 
manufactures  green  pigment  instead  of  red,  cellulose  in- 
stead of  lignin,  or  any  other  substance,  at  the  surface, 
and  makes  the  cell-wall  of  even  resistance  to  the  osmotic 
pressure  within,  thus  producing  a  sphere  and  not  an  ellip- 
soid, or  filament,  or  any  other  shape. 

38.  What  is  Inheritance.—  When  the  Pleurococcus  cell 
divides,  this  wonderful,  invisible  mechanism  —  the  certain 


IlKRKDITY 


47 


definite  physical  and  chemical  constitution — is  transmitted 
to  each  of  the  daughter-cells;  each,  in  other  words,  re- 
ceives Pleurococcus  protoplasm.  This  protoplasm,  with 
its  definite  organization,  constitutes  the  inheritance.  The 
daughter-cells  do  not  inherit  a  spherical  shape  (as  is  evident 
from  Fig.  33),  but  a  definite  kind  of  protoplasm,  cell-sap 


FIG.  34. — Pleurococcus  vulgaris.  Sections  of  one-,  two-,  and  four-celled 
plants,  showing  the  nuclei  and  the  large  chlorophyll  bodies  (chb)  to  which 
the  green  color  of  the  plants  is  due.  In  D,  the  larger  chloroplast  is  shown 
in  perspective.  (Camera  lucida  drawings  from  a  microscopic  preparation 
by  E.  W.  Olive.).  (Cf.  Fig.  33.) 

of  certain  osmotic  properties,  and  surface  cellulose  of  even 
elasticity,  so  that,  in  surroundings  uniform  on  all  sides, 
a  spherical  shape  must  finally  result.  The  shape  is  an 
expression  of  the  inheritance  for  the  given  environment. 
Under  different  external  conditions  the  expression  might 
be  different;  but  the  inheritance  would  be  the  same.  The 
chlorophyll  in  the  daughter-cells,  immediately  after  cell- 


48  HEREDITY   AND    EVOLUTION  IN   PLANTS 

division,  is  a  direct  inheritance,  but  the  chlorophyll  subse- 
quently manufactured,  and  the  green  color  which  it  gives 
to  the  plant,  are  not  inherited;  they  are  expressions  of  the 
inheritance — which  in  this  instance  is  a  chloroplastid 
(Fig.  34)  that  reproduces  itself  by  division,  and  manufac- 
tures chlorophyll  in  the  presence  of  sunlight.  Under  abnor- 
mal external  conditions  the  mechanism  may  not  act,  or 
may  act  abnormally,  so  that  yellow  pigment  appears 
instead  of  green — or  in  darkness  no  pigment  at  all.  In 
either  case  the  inheritance  is  the  same,  but  the  expression 
varies.  A  modern  writer  (J.  Arthur  Thomson)  has  denned 
inheritance  as  all  that  an  organism  has  to  start  with.  It  is 
the  protoplasmic  substance,  with  all  its  potentialities, 
passed  on  from  parent  to  offspring. 

39.  Inheritance  Versus  Expression.— In  the  light  of 
this  information,  obtained  by  a  study  of  the  simple  Pleuro 
coccus,  we  are  able  to  understand  that  what  we  inherit 
from  our  parents  or  grandparents,  is  not  a  certain  shape  of 
nose,  a  certain  characteristic  gait,  a  musical  or  mathe- 
matical bent  of  mind,  a  quick  temper,  but  a  substance 
(protoplasm)  possessing  a  very  delicate,  intricate,  and 
characteristic  constitution  or  mechanism.  Under  certain 
conditions  this  inheritance  may  so  express  itself  as  to 
cause  resemblance  in  some  physical  or  mental  trait;  or  it 
may  find  a  quite  different  expression,  as  when  parents  of 
medium  height  have  tall  children,  or  parents  musically 
inclined  have  children  that  do  not  care  for  music;  or  sweet- 
peas,  having  white  flowers  only,  produce,  when  crossed, 
peas  having  colored  flowers.  Or  again,  not  all  that  is  in- 
herited may  be  expressed;  this  is  illustrated  when  children 
resemble,  not  their  parents,  but  their  grandparents. 
Here  the  parents  transmitted  an  inheritance  which,  in 
them,  found  no  expression. 


HEREDITY 


40 


A  remarkable  illustration  of  inheritance  without  expres- 
sion is  seen  in  the  case  of  the  alternation  of  generations 
(pages  33-35).  The  inital  protoplasm  of  the  sporophyte 
is  all  inherited  through  the  fertilized  egg  from  the  game- 


FIG.  35. — Vegetative  propagation  of  Haworthia  sp.  The  new  plantlet 
forms  on  the  flower  stalk,  below  the  flower-cluster.  Ultimately  it  falls 
to  the  ground  and  takes  root,  becoming  established  as  an  independent 
plant. 

tophytes,  but  most  of  the  gametophytic  characters  do  not 
appear  in  the  sporophyte,  nor  do  the  typically  sporophytic 
characters  find  expression  in  the  gametophy  te. l  (Cf.  p .  40 .) 

1  The  chlorophyll,  of  course,  is  an  exception.  But  the  osmotic  strength 
of  the  cell-sap  is  a  different  expression  in  gametophyte  and  sporophyte, 
otherwise  the  young  sporophyte  could  not  live  parasitically  upon  the 
gametophyte. 


50  HEREDITY   AND   EVOLUTION   IN   PLANTS 

40.  Inheritance  Versus  Heredity. — As  stated  above,  the 
inheritance  is  that  which  is  actually  transmitted  from  parent 
to  offspring.     The  fern-spore,  for  example,  is  the  inheri- 
tance  of   the   fern   gametophyte   from   the   sporophyte. 
Heredity  is  the  genetic  relationship  that  exists  between  suc- 
cessive generations  of  organisms.     The  relation  between  two 
brothers  and  their  parents  is  similar — it  is  one  of  heredity; 
their  inheritance  may  be  quite  different. 

41.  Inheritance  and  Reproduction. — Inheritance  is,  of 
course,  inseparably  linked  with  reproduction  and  may  be 
studied  in  connection  with  the  three  following  types : 

1.  In  vegetative  propagation,  e.g.,  by  means  of  tubers, 
cuttings  and  "slips,"  bulbs  and  bulbils,  gemmae,  "run- 
ners," scions,  vegetative  rejuvenation  or  "budding"  (Fig. 
35),  etc.,  the  new  plant  is  obviously  only  a  portion  of  the 
vegetative  tissue  of  the  parent  plant,  iso  lated  and  growing 
independently  by  itself.     The  separationof  the  propagating 
piece  from  the  parent  is  often  (though  not  always)  mechan- 
ical and  artificial.     The  protoplasm  remains  unaltered  by 
the  act  of  separation,  reduction  divisions  of  cell-nuclei  are 
not  involved,  and  the  inheritance,  except  in  bud-varia- 
tions, is  unaffected  by  the  change.     This  is  evident  in  those 
cases  where  the  isolated  piece  is  grafted  upon  another 
plant;  the  specific  or  varietal  characteristics  of  the  scion 
are  seldom,  if  ever,  affected  by  the  stock.     Thus,  in  the 
experiment  illustrated  in  Fig.   36,   a  tomato  stem  was 
grafted  upon  a  tobacco  plant,  and  upon  the  tomato  were 
grafted  three  other  species — Solanum  nigrum,  Solanum 
integrifolium,  and  Phy sails  Alkekengi.     Each  species  was 
apparently  not  in  the  least  altered  by  this  drastic  change 
in  the  conditions  of  its  life. 

2.  In  asexual  reproduction  by  spores  the  situation  is 
quite  similar  to  that  in  vegetative  propagation,  but  in 


HEREDITY 


certain  cases  there  is  abundant  opportunity  tor  the  proto- 
plasm to  become  more  or  less  altered  during  the  compli- 
cated changes  that  accompany  the  division  of  the  cell- 
nucleus.^  This  is  notably  the  case  in  the  chromosome  re- 


FIG.  36. — Graft  of  tomato  (Lycopersicum  esculentum)  on  tobacco 
(Nicotiana  tabacum).  On  the  tomato  are  grafted  Solanum  nigrum,  S 
integrifolium,  and  Physalis  Alkekengi.  (Graft  made  by  Mr.  M.  Free.) 

duction  divisions  preceding  spore-formation  in  the  sporo- 
phytes  of  higher  plants  (p.  37) ,  especially  when  the  plant  is 
a  hybrid;  and  in  spore-formation  in  the  sporangia  produced 
from  the  zygospore  of  some  of  the  filamentous  fungi,  like 
Rhizopus  or  Mucor,  the  common  black  mold  of  bread.  In 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


the  latter  case  the  nuclear  divisions,  some  time  preceding 
spore-production,  result  in  separating  out  the  female  (+) 
and  male  (— )  strains,  so  that  the  spores  in  a  given  sporan- 
gium are  unlike  as  to  sex — some  being  female  (+),  some 
male  (— ),  (Fig.  37).  This  will  be  discussed  more  fully 
in  the  next  chapter.  Such  changes  result  merely  in  dis- 
tributing the  heritable  units  (genes)  of  the  mother-cell 


FIG.  37. — Sexual  reaction  between  a  hermaphroditic  Mucor  and  (+) 
and  (  — )  races  of  a  dioecious  species.  Diagrammatic  representation  of  a 
Petri  dish  culture  showing  a  heterogamic  hermaphroditic  Mucor  (^!)  in 
the  center  separated  by  channels  on  either  side  from  the  (+)  and  (  — ) 
races,  respectively,  of  a  dioecious  species.  Sp.,  sporangia  containing 
spores  by  means  of  which  the  plant  may  be  reproduced  nonsexually. 
1-6,  stages  in  development  of  a  hermaphroditic  zygospore  from  unequal 
male  and  female  gametes.  A,  sexual  reaction  between  a  (  — )  filament  and 
a  female  gamete.  B,  sexual  reaction  between  a  (+)  filament  and  a  male 
gamete.  C,  a  male  zygospore  formed  by  stimulus  of  contact  with  a  (+) 
filament.  (After  Blakeslee.) 

unequally  to  the  daughter-cells,  but  introducing  nothing 
new;  they  may,  however,  result  in  the  complete  loss  of 
one  or  more  heritable  units,  or  in  the  formation  of  a  new 
one,  not  existent  in  the  parent.  In  the  latter  two  cases 
we  recognize  a  mutation.  No  hard  and  fast  line  can  be 
drawn  between  the  various  kinds  of  asexual  reproduction; 
there  are  various  degrees  of  transition  between  reproduction 


HEREDITY  53 

by  spores,  gemmae,  bulbs  and  tubers,  and  the  artifically 
severed  buds  and  scions  used  in  grafting  and  "slipping." 
3.  In  sexual  reproduction  there  intervene  between  par- 
ents and  offspring,  not  only  the  complicated  reduction 
divisions  involved  in  the  formation  of  the  gametes,  but 
also  the  nuclear  and  cell-fusions  accomplished  by  the  union 
of  the  egg  and  sperm  in  fertilization  (Fig.  38).  Both  proc- 
esses— the  formation  of  the  gametes,  and  their  fusion — 


FIG.  38. — Fertilization  in  the  white  pine  (Finns  Strobus).  The  smaller 
sperm-nucleus  (above)  is  imbedded  in  the  (larger)  egg-nucleus.  The  fu- 
sion of  the  nucleoplasms  will  finally  become  more  intimate.  (After 
Professor  Margaret  C.  Ferguson.) 

offer  almost  unlimited  opportunities  for  alterations  of  the 
protoplasm— especially  that  of  the  nucleus.  This  method 
of  reproduction,  therefore,  has  the  very  greatest  interest 
and  importance  for  the  study  of  heredity.  In  the  fertilized 
egg1  are  united  inheritances  from  two  parents — from  two 
distinct  lines  of  ancestry — protoplasms  (germ-plasms)  with 
two  entirely  different  histories  extending  back  into  the 

1  The  fertilized  egg  (as  Thomson  has  pointed  out)  is  the  inheritance, 
and  becomes,  in  the  mature  individual,  the  inheritor. 


54  HEREDITY   AND   EVOLUTION   IN   PLANTS 

past,  no  one  knows  how  far.  How  will  these  two  inheri- 
tances affect  each  other  when  they  intermingle  in  the 
fertilized  egg?  Will  one  tend  to  inhibit  or  check  certain 
characteristics  or  functions  of  the  other;  will  they  evenly 
blend,  so  as  to  produce  an  expression  intermediate  between 
that  of  the  parents;  or  may  entirely  new  substances  be 
formed  or  new  combinations  take  place,  resulting  in  an  en- 
tirely new  expression  in  the  offspring? 

42.  Methods  of  Study. — To  endeavor  to  answer  the 
questions  just  asked  is  as  fascinating  an  occupation  as  it  is 
important,  and  the  answers  are  significant  for  man,  as  well 
as  for  plants.  It  is  indeed,  a  fortunate  thing  that  prin- 
ciples ascertained  by  studying  plants  apply  equally  to  man 
and  other  animals,  since  plants  are  so  much  easier  to 
handle  in  experimental  investigations. 

We  may  go  about  the  answering  of  these  questions  in 
either  of  two  ways.  We  may  gather  large  numbers  of 
statistics  to  measure  and  analyze  (statistical  or  biometrical 
method),  or  we  may  employ  the  experimental  method.  The 
method  of  biometry  enables  us  to  deal  with  a  larger  number 
of  individuals,  but  the  material  studied  is  usually  a  mixed 
population,  whose  history  is  only  imperfectly  known,  the 
conditions  are  more  complex,  and  little  if  at  all  under 
control.  By  the  experimental  method  it  is  not  necessary 
to  deal  with  such  large  numbers;  we  may  choose  carefully 
pedigreed  material,  about  the  history  of  which  we  have 
more  or  less  accurate  knowledge,  and  we  may  greatly 
simplify  and  control  the  conditions  under  which  we  make 
our  observations.  The  largest  advance  toward  the  solu- 
tion of  the  problems  of  inheritance  has  been  made  by  the 
experimental  method,  in  the  form  first  employed  success- 
fully by  Gregor  Mendel.  This  method  will  be  briefly 
explained  in  the  next  chapter. 


CHAPTER  V 
EXPERIMENTAL  STUDY  OF  HEREDITY 

43.  Gregor   Mendel. — Two    of    the    most    important 
contributions  ever  made  to  biological  science,   namely, 


FIG.  39. — Gregor  Mendel,  at  the  age  of  40.  His  theory  of  alternate 
inheritance  (Mendelism),  based  largely  on  experiments  with  the  garden 
pea,  is  the  most  important  and  most  fruitful  contribution  ever  made  to 
the  study  of  inheritance. 

Mendel's  laws  of  heredity,  and  his  method  of  investigating 
them,  were  made  by  a  teacher  who  studied  plants  as  a  pas- 
time because  he  loved  to  do  it.  This  man  was  Gregor 

55 


56  HEREDITY   AND    EVOLUTION   IN  PLANTS 

Mendel,  a  monk  in  the  monastery  at  Briinn,  Austria,  where 
he  finally  became  abbott.  In  order  to  understand  his  work 
clearly  the  student  should  familiarize  himself  the  various 
characters  of  the  edible  or  garden  pea,  the  chief  plant  with 
which  Mendel  worked. 

44.  Mendel's  Problem. — Mendel  was  much  interested 
in  problems  concerning  the  origin  and  evolution  of  species. 
It  was  largely  this  interest  that  led  him  to  hybridize  (i.e., 
cross-pollinate)  plants  of  different  species  and  varieties, 
and  observe  the  behavior  of  the  resulting  hybrids  in  succes- 
sive generations.     The  problem  which  he  endeavored  to 
solve  was  the  law  or  laws  "governing  the  formation  and 
development  of  hybrids,"1  with  special  reference  to  the 
laws  according  to  which  various  characters  of  parents 
appear  in  their  offspring. 

45.  Mendel's  Method. — He  recognized  that,  in  order 
to  solve  the  problem,  attention  must  be  given  to  at  least 
four  points,  as  follows: 

1.  To  start  with  pure-breeding  strains. 

2.  To  consider  each  character  separately. 

3.  To  keep  the  different  generations  distinct. 

4.  To  record,  for  the  progeny  of  each  generation  sepa- 
rately, the  proportions  in  which  the  various  characters 
appear. 

No  previous  student  had  recognized  the  fundamental 
importance  of  these  requirements. 

46.  Choice    of   Material. — Mendel    realized    that  the 
success  of  any  experiment  depends  upon  choosing  the 
most  suitable  material  with  which  to  experiment.     He 
laid  down  the  requirements  as  follows : 

-1  All  the  quotations  in  this  chapter  are  from  an  English  translation  of 
Mendel's  original  paper.  His  form  of  expression  has  been  preserved  as 
far  as  possible,  even  when  the  "quotes"  are  omitted. 


EXPERIMENTAL   STUDY   OF   HEREDITY  57 

1.  "The  experimental  plants  must  necessarily  possess 
constant  differentiating  characters."1 

2.  "The  hybrids  of  such  plants  must,  during  the  flower- 
ing period,  be  protected  from  the  influence  of  all  foreign 
pollen,  or  be  easily  capable  of  such  protection. 

3.  "The  hybrids  and  their  offspring  should  suffer  no 
marked  disturbance  in  their  fertility  in  the  successive 
generations." 

Mendel  also  called  attention  to  the  advantage  of  choos- 
ing plants  which,  like  the  peas,  are  easy  to  cultivate  in 
the  open  ground  and  in  pots,  and  which  have  a  relatively 
short  period  of  growth. 

47.  Characters  Chosen  for  Observation. — "Each  pair 
of  differentiating  characters  [have  been  thought  to]  unite 
in  the  hybrid  to  form  a  new  character,  which  in  the  pro- 
geny of  the  hybrid  is  usually  variable.  The  object  of  the 
experiment  was  to  observe  these  variations  in  the  case  of  each 
pair  of  differentiating  characters,  and  to  deduce  the  law  ac- 
cording to  which  they  appear  in  successive  generations.  The 
experiment  resolves  itself  therefore  into  just  as  many 
separate  experiments  as  there  are  constantly  differentia- 
ting characters  presented  in  the  experimental  plants." 
The  following  were  the  characters  chosen  for  observation  : 

1.  The  difference  in  the  shape  of  the  ripe  seeds  (round 
and  smooth  vs.  angular  and  wrinkled). 

2.  The  difference  in  the  color  of  the  cotyledons  (pale 
or  bright  yellow,  or  orange  vs.  light  or  dark  green). 

1  Differentiating  characters  are  those  in  respect  to  which  the  two  species 
or  varieties  to  be  crossed  differ.  The  possession  of  chlorophyll  by  the 
leaves  of  peas,  for  example,  is  a  common  character.  "  Common  characters 
are  transmitted  unchanged  to  the  hybrids  and  their  progeny."  The  color 
of  the  corolla  (for  example,  white  in  one  species  and  purple  in  the  other)  is 
a  differentiating  character,  serving  to  differentiate  or  distinguish  one  species 
from  another. 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


3.  The  difference  in  the  color  of  the  seed-coat  (white 
vs.  gray,  gray-brown,  leather-brown,  with  or  without  violet 
spotting,  etc.). 

4.  The  difference  in  the  form  of  the  ripe  pods  (deeply 
! M<>  f . — -j    constricted  between  the  seeds  and 

more    or    less    wrinkled,    or   the 
opposite) . 

5.  The  difference  in   the   color 
of  the  unripe  pods  (light  or  dark 
green  vs.  vivid  yellow). 

6.  The  difference   in   the  posi- 
tion of  the  flowers  (i.e.,  axial  vs. 
terminal,  on  normal  vs.  fasciated 
stems). 

7.  The  difference  in  the  length 
of  the  stem  (the  extremes  chosen 
were   "tails"    6    to    7    feet,    and 
"dwarfs"  Y±  feet   to  i>£  feet  in 
height). 

48.  Artificial  Hybridizing. — 
The  edible  pea  is  commonly  self- 
fertilized;  therefore,  to  make 
crosses  it  is  necessary  carefully  to 

remove  the  stamens  of  one  flower 
FIG.  40. — Method  of  pro- 
tecting flowers  from  foreign  before  the  anthers  have  begun  to 
pollen    by  paper  bags,  in  shed  their  pollen,  and  then  place 

P°Uen  fr°m  another  nower-on  the 
stigma.  The  flowers  must  then 
be  carefully  guarded,  e.g.,  by  tying  paper  bags  over  them 
(Fig.  40),  to  prevent  other  pollen  being  deposited  by 
insects  or  otherwise.  In  this  way  the  experimenter 
knows  just  what  characteristics  enter  into  the  hybrid. 


EXPERIMENTAL    STUDY   OF    HEREDITY  59 

Careful  record  is  kept  of  all  data,  and  plants  produced  in 
this  way,  with  ancestral  characters  noted  and  .recorded, 
are  called  pedigreed.  Plantings  of  such  plants  are  called 
pedigreed  cultures. 

In  many  species,  in  "making  the  cross"  (i.e.,  doing  the 
cross-pollinating)  great  care  must  be  taken  to  avoid  con- 
tamination from  foreign  pollen,  of  which  the  air  may  be 
full.  The  fingers  and  all  instruments  are  usually  rinsed 
in  alcohol  before  each  operation,  to  insure  killing  any 
foreign  pollen  that  might  be  present.  Numerous  other 
precautions  are  also  taken. 

When  the  hybrid  plants  are  mature,  careful  observations 
of  whatever  character  is  under  observation  are  made  and 
recorded.  Whenever  possible  the  observation  should  be 
quantitative. 

49.  Mendel's  Discoveries. — We  may  illustrate  Men- 
del's results  in  a  simple  manner  by  choosing,  as  the  pair 
of  contrasted  characters,  smooth  and  wrinkled  seeds  of  the 
pea.  Removing  all  the  stamens  from  flowers  of  a  variety 
having  smooth  seeds,  he  pollinated  those  flowers  with 
pollen  from  a  plant  bearing  wrinkled  seeds. 

It  should  now  be  kept  clearly  in  mind  just  what  the 
inheritance  of  the  fertilized  egg  is  in  such  a  case.  From 
the  pistillate  plant  the  inheritance,  contributed  by  the 
egg-cell,  included  the  protoplasmic  properties  (whatever 
they  may  be)  which,  when  free  to  produce  their  effect, 
cause  smooth  seeds;  from  the  staminate  parent  the  in- 
heritance, contributed  by  the  sperm-cell,  included  the 
protoplasmic  properties,  which,  when  free  to  act,  cause 
wrinkled  seeds. 

i.  Law  of  Dominance. — What  Mendel  actually  found 
by  his  experiments  was  that,  in  such  a  cross,  all  the  seeds 


60  HEREDITY   AND   EVOLUTION   IN   PLANTS 

of  the  hybrid  plants  are  smooth.  The  inheritance  was 
"smooth"  and  "wrinkled,"  but  the  expression  was  of 
only  one  type — smooth.  A  character  thus  expressed,  to 
the  exclusion  of  another,  in  the  first  filial  (Fi)  genera- 
tion Mendel  called  dominant,  and  the  phenomenon  he 
called  dominance;  the  other  character  is  recessive.  From 
such  observations  Mendel  formulated  the  law  of  domi- 
nance, as  follows:  When  pairs  oj  contrasting  characters 
are  combined  in  a  cross,  one  character  behaves  as  a  dominant 
over  the  other,  which  is  recessive. 

By  similar  experiments  Mendel  found  that,  in  the  coty- 
ledons, yellow  is  dominant  over  green,  tallness  over  dwarf- 
ness,  axial  flowers  over  terminal,  and  so  on.  Such  pairs 
of  contrasting  characters  are  called  allelomorphs. 

2.  Law  of  Segregation. — But  what  will  happen  if  the 
first  filial  (Fi)  generation  is  inbred  or  self-pollinated.  Its 
inheritance  included  factors  that  make  for  both  "smooth" 
and  "wrinkled,"  but  the  expression  was  of  one  kind  only. 
The  experiment  was  made,  and  Mendel  found  that  the 
second  filial  (F2)  generation  included  plants,  part  of  which 
possessed  only  smooth  seeds,  while  the  others  had  only 
wrinkled  seeds  (Fig.  41).  "Transitional  forms  were  not 
observedin  any  experiment."  This  illustratesin  a  striking 
way  the  difference  between  inheritance  and  expression, 
for  a  character  cannot  appear  in  a  plant  (or  animal)  unless 
the  plant  possesses  the  factor  or  factors  for  that  character. 
Now,  except  for  the  comparatively  rare  cases  where 
mutation  occurs,  the  factors  in  the  F2  generation  must  have 
been  derived  by  inheritance  from  the  germ-cells  of  the  FI 
generation;  but  the  experiment  shows  that  they  did  not 
come  to  expression  there.  The  same  law  is  illustrated  in 
the  crossing  of  a  sweet  variety  of  maize  (having  wrinkled 


EXPERIMENTAL   STUDY   OF   HEREDITY 


6l 


grains)  with  a  starchy  variety  (hav'ng  smooth  grains) .     In 
this  cross  starchiness  is  dominant  over  sweetness  (Fig.  42). 


II 


I! 


i 


FIG.  41. — Mendelian   segregation   in  the  edible  pea  (Pisum  sativitm) 
Full  explanation  in  the  text.     (Cf.,  Fig.  42.) 

50.  Ratio  of  Segregation. — But  now  we  come  to  that 
feature  of  Mendel's  experiments  which,  perhaps  more  than 


HEREDITY   AND    EVOLUTION   IN   PLANTS 


FIG.  42. — Mendelian  segregation  in  maize,  a,  the  starchy  parent;  b, 
the  sweet  parent;  C,  the  first  hybrid  (FO  generation,  produced  by  crossing 
a  and  b,  showing  the  dominance  of  starchiness;  d,  the  second  hybrid  (F2) 
generation,  showing  the  segregation  of  starchiness  and  sweetness  with  the 
ratio  of  three  starchy  to  one  sweet  (wrinkled)  grain.  Lower  row,  daughters 
of  d;  e,  f,  and  g  resulted  from  planting  starchy  grains.  One  ear  in  three  is 
pure  starchy,  the  other  two  mixed;  h,  result  of  planting  sweet  (wrinkled) 
seed.  They  are  pure  recessives,  and  the  ear  is  pure  sweet.  (After 
East.)  (Cf.  Fig.  41.) 


EXPERIMENTAL   STUDY   OF   HEREDITY  63 

anything  else,  made  them  superior  to  all  others  that  had 
preceded.  He  carefully  counted  the  number  of  plants 
bearing  each  kind  of  seed,  and  found  that  the  number 
of  smooth-seeded  plants  was  to  those  with  wrinkled 
seeds  as  3  :  i. 

51.  Theory  of  Purity  of  Gametes. — When  the  wrinkled 
seeds  (one-fourth  of  the  total  crop)  were  sown  they  all 
bred  true  to  wrinkledness — their  descendants  of  the  F$ 
generation  bearing  only  wrinkled  seeds.     The  expression 
was  alike  in  every  case.     The  gametes  that  united  to 
produce  these  plants  were  therefore  considered  pure  for 
"wrinkledness;"  that  is,  it  was  inferred  that  they  did  not 
carry  any  inheritance  tending  to  produce  smoothness  of 
seed. 

52.  Not  All  Dominants  Alike.— But  when  the  seeds  of 
the  F2  plants,  having  only  smooth  seeds,  were  sown  it 
was  found  that  the  dominants  were  not  alike,  except  in 
external   appearance.     The  seeds,   though  all  appeared 
smooth,     carried    different    inheritances.     One-third    of 
them  (i.e.,  one-fourth  of  all  the  seed  produced  by  the  F2 
generation)  bred  true  to  smoothness,  being  therefore  pure, 
or  homozygous  for  smoothness;  the  other  two-thirds  of 
the  dominants  (i.e.,  one-half  of  all  the  seed  produced) 
again  segregated  in  the  ratio  of  3:1 — one-fourth  wrinkled 
and  three-fourths  smooth,  showing  that  they  were  hetero- 
zygous; that  is,  that  they  still  carried  inheritance  from 
both  the  wrinkled  and  smooth-seeded  grandparents. 

If  we  designate  the  first  parental  generation  by  P,  the 
dominant  character  (whatever  it  may  be)  by  D,  and  the 
recessive  character  by  R,  then  the  facts  above  described 
may  be  diagrammed  as  follows : 


64  HEREDITY   AND   EVOLUTION   IN   PLANTS 

D9  X  Rrf1  P      (ist  Parental  generation) 

D  (R)          Fi     (ist  Hybrid  generation) 


3D  iR     F»     (2d  Hybrid  generation) 


iD 


D         3D       iR    R      Fi     (3d  Hybrid  generation) 

53.  Significance  of  the  Mendelian  Ratio.—  The  ratio 
3  :  i  or,  as  it  appears  on  analysis,  i  :  2  :  i,  is  the  ratio  that 
one  might  expect,  or  that  might  be  predicted,  on  the  basis 
of  chance.     Students  of  algebra  will  recognize  in  it  the 
essence  of  the  familiar  square  of  a  +  b,  namely,  a2  + 
2ab  +  b~,  where  a  and  b  each  equal  i.     In  the  plants  the 
multiplication  of  inheritances  (produced  in  fertilization) 
was  as  follows  : 

eggs  (s  +  w)  X  sperms  (s  +  w)  =  ss  +  2sw  +  ww 

where  w  =  wrinkling  and  s  =  absence  of  wrinkling,  i.e., 
smoothness. 

54.  Theory  of  Purity  of  Gametes.  —  The  above  ratio 
is  what  we  would  expect  if  half  of  the  egg-cells  and  half 
of  the  sperm-cells  in  a  heterozygous  plant  (one  of  the  FI 
generation),  carried  only  character-units  or  determiners1 
that  make  for  smoothness;  the  other  half  only  those 
factors  that  make  for  wrinkling,  giving  s  and  w  egg-cells, 
and  5  and  w  sperm-cells  in  equal  numbers.     Therefore,  in 
pollination  the  chances  would  be  equal  that  an  s-egg  would 

1  The  substance  or  condition  (protoplasmic  constitution),  whatever  it  is, 
in  the  germ-cells  that  corresponds  to  any  given  character  of  the  plant  is 
variously  referred  to  by  the  terms  factor,  determiner,  gene  (=  producer), 
character-unit,  and  others.  These  terms  are  essentially  synonyms. 


EXPERIMENTAL   STUDY   OP   HEREDITY  65 

be  fertilized  with  either  an  s-sperm  or  a  w-sperm,  giving 
(s  +  w)  X  (s  +  w)  =  55  +  25W  +  w«>.  Since  5  is  dominant 
over  w  the  product  should  be  written  : 


giving  in  external  appearances  35  +  iw.  Since  the  re- 
sult actually  observed  is  what  it  would  be  */  the  gametes 
were  thus  "pure"  for  smoothness  and  wrinkling,  Mendel 
concluded  that  they  really  are,  and  moreover  that  each 
character  behaves  as  a  unit,  appearing  and  disappearing 
in  its  entirety. 

55.  Character-units  versus  Unit-characters.  —  As  just 
stated,  Mendel  held  that  the  various  visible  characters  of 
his  plants  (dwarfness,  for  example)  behaved  as  units, 
either  appearing  in  their  fullness,  or  not  appearing  at  all. 
From  more  careful  observations  we  know  that  such  is 
not  the  case.  A  blossom  may,  for  example,  be  more  or 
less  pink,  an  odor  more  or  less  strong,  dwarfs  are  not 
all  the  same  height,  but  fluctuate  around  a  mean.  We 
conclude  therefore  that  characters  do  not  behave  as 
units,  and  that  the  conception  of  "unit-characters"  is 
erroneous.  The  evidence  does,  however,  seem  to  justify 
the  conclusion  that  the  factor  or  factors,  whatever  they 
may  be,1  that  are  causally  related  to  the  given  character 
do  behave  as  units.  We  may  therefore  designate  them 
as  character-units.  Since  they  are  causally  or  genetically 
related  to  the  character  they  have  been  called  genes  (from 
the  root  of  the  Greek  word,  genesis).  They  are  more 
commonly  known  as  factors.  Quite  probably,  in  many 
if  not  all  cases,  more  than  one  factor  is  involved  in  the 
production  of  any  given  character. 

1  Substance  or  condition,  we  know  not  what,  within  the  germ-cells. 


66  HEREDITY   AND    EVOLUTION   IN  PLANTS 

56.  Applications  of  Mendel's  Law. — Over  100  pairs 
of  structural  and  color  characters  have  been  found,  in 
plant  breeding,  to  behave  more  or  less  closely  in  accord- 
ance with  the  Mendelian  conception.     In  peas  alone  over 
20  pairs  of  characters  are  expressed  in  successive  genera- 
tions, in  accordance  with  this  law.     Among  the  more 
striking  results  which  are  explainable  upon  Mendelian 
theory  are  the  following : 

1.  Mottled  beans  have  been  produced  in  the  FI  genera- 
tion by  crossing  two  varieties,  neither  of  which  had  mottled 
seeds.     Various   types    appeared   in   the  F2  generation. 

2.  Jet  black  beans  have  appeared  in  the  FI  generation 
from  a  cross  between  two  varieties,  one  of  which  had  pure 
white  seeds,  the  other  light  yellow.     Various  shades  and 
colors  appeared  in  the  F2  generation. 

3.  In  one  case  three  distinct  varieties  of  beans,  breed- 
ing true  to  white  seeds  (when  self ed1),  were  crossed  with  the 
same  variety  of  red  bean.     In  the  FI  generation  each  cross 
gave  a  different  color — one  blue,  another  black,  and  the 
third  brown.     A  varied  assortment  of  colors  appeared  in 
each  case  in  the  F2  generations. 

4.  Two  varieties  of  sweet  peas,  each  breeding  true  to 
white  flowers,  when  crossed  gave,  in  the  FI  generation, 
nothing   but   purple-flowered    offspring,    resembling    the 
wild  sweet  pea.     A  medley  of  white,  pink,  purple,  and 
red-flowered  plants  appeared  in  the  F2  generation.     Num- 
erous other  cases  might  be  cited,  all  of  which  would  have 
been  unsolvable  riddles  except  in  the  light  of  Mendelism. 

57.  Inheritance  and  Environment. — Emphasis  should 
be  laid  on  the  fact  that  the  behavior  of  any  plant,  and  the 

1  The  pollination  of  a  flower  with  its  own  pollen,  or  with  pollen  from  an- 
other flower  of  the  same  plant,  is  called  sdfing. 


EXPERIMENTAL    STUDY   OF   HEREDITY  67 

characters  it  manifests,  are  the  result  of  the  combined 
influence  of  inheritance  and  environment.  A  bean  seed- 
ling^is  green,  not  merely  because  it  has  inherited  chloro- 
plastids,  but  because  it  develops  in  sunlight;  without 
sunlight  the  green  color  could  not  come  into  expression. 
If  we  vary  any  factor  of  environment  (temperature,  mois- 
ture, illumination,  food)  the  expression  of  the  inheritance 
may  be  altered,  just  as  truly  as  though  the  inheritance 
were  changed.  The  characteristics  expressed  by  any  plant 
(or  animal)  are  the  result  of  the  combined  action  of  inheri- 
tance and  environment.  It  is  of  fundamental  concern  to 
a  man,  not  only  to  be  "well-born"  (eugenics),  but  also 
to  be  "well-placed"  (euthenics),  although  the  former, 
according  to  present  day  conceptions  appears  to  be  more 
important. 

58.  Johannsen's  Conception  of  Heredity. — The  con- 
ception that  inheritance,  as  previously  noted,  is  not  the 
transmission  of  external  characters  from  parent  to  off- 
spring, but  the  reappearance,  in  successive  generations, 
of  the  same  organization  of  the  protoplasm  with  reference 
to  its  character-units,  was  first  developed  by  Johannsen, 
of  Copenhagen,  Denmark,  who  proposed  the  term  "genes." 
"The  sum  total  of  all  the  'genes'  in  a  gamete  or  zygote," 
is  a  genotype.  Inheritance  is  the  recurrence,  in  successive 
generations,  of  the  same  genotypical  constitution  of  the  pro- 
toplasm. Johannsen  does  not  attempt  to  explain  the 
nature  of  the  genes,  "but  that  the  notion  'gene'  covers 
a  reality  is  evident  from  Mendelism."  This  conception 
of  heredity  is  diametrically  opposed  to  the  older  and 
popular  conception,  but  is  much  more  closely  in  accord 
with  the  facts  revealed  by  recent  studies  of  plant  and 
animal  breeding  (Cf.  p.  40). 


68  HEREDITY   AND   EVOLUTION   IN   PLANTS 

59.  Pure  Line  Breeding. — Johannsen  also  originated 
the    "pure    line"    theory — a    theory    which    has    done 
much  toward  elucidating  the  problems  of  selection.     He 
and  his  followers  regard  genetic  factors  as  fixed  and  un- 
varying.    Hence  the  results  obtained  in  selective  breeding 
of  a  given  variety  of  maize  for  high  or  low  oil  content,  or 
of  a  given  variety  of  beans  for  larger  or  smaller  size  of  seed, 
would  be  interpreted  on  this  theory,  as  the  isolation  or 
separation  of  pure  strains  from  a  "mixed  population" 
or  "impure"  variety.     In  practical  language,  several  true 
breeding  varieties  of  beans,  differing  in  seed  size,  might  be 
obtained  by  selection  from  what  appeared  to  be  a  "pure" 
variety  with  considerable  variation  in  size  of  seeds.1 

60.  Value  of  Mendel's  Discoveries. — The  discoveries 
that,  in  inheritance,  certain  characters  are  dominant  over 
certain  others;  that  a  given  inheritance  (e.g.,  conditions 
associated  with  seed-color,  odor,  eye-color,  stature,  musi- 
cal ability,  insanity,  tendency  to  some  disease)  may  be 
carried  and  transmitted  to  offspring  by  an  adult  who  gives 
no  outward  sighs  of  carrying  the  inheritance;  that,  under 
certain  conditions  of  breeding,  some  characters  (the  re- 
cessive ones),  whether  good  or  bad,  may  become  perma- 
nently lost;  that  dominant  characteristics  are  certain  to 
reappear  in  some  of  the  offspring — all  of  these  truths, 
learned  by  the  study  of  a  common  garden  vegetable,  will 
be  recognized  at  once  as  of  enormous  importance  to  the 
breeders  of  plants  and  animals,  and  above  all  to  man- 
kind, in  connection  with  our  own  heredity.     They  point 
the  way  to  the  explanation  of  such  enigmas  as  the  pro- 
verbial bad  sons  of  pious  preachers,  spendthrift  children 

1  A  detailed  discussion  of  Johannsen's  method  of  "pure  line"  breeding 
belongs  to  more  advanced  studies. 


EXPERIMENTAL   STUDY   OF    HEREDITY  69 

of  thrifty  parents,  talented  offspring  of  mediocre  parents, 
blue-eyed  children  of  brown-eyed  parents,1  and  so  on. 

61.  Increased  Vigor  from  Crossing.— Experiments  with 
pedigreed  cultures  have  disclosed  a  principle  of  the  utmost 
practical  importance  for  the  plant  bree'der.  A  careful 
analysis  of  a  field  of  Indian  corn  (Zea  Mays)  has  disclosed 
the  fact  that  any  given  variety  is  very  complex,  being 
heterozygous  for  many  characters;  in  other  words  any 
horticultural  variety  is  a  composite  of  numerous  elemen- 
tary species,  and  is  therefore  heterozygous  for  most  of  its 
characters.  When  pollination  is  allowed  to  take  place  in 
the  corn  field  without  interference  by  man,  both  crossing 
and  selfing  occur.  As  a  result  the  yield,  in  bushels  per 
acre,  remains  about  stationary,  or  gradually  becomes  less 
and  the  variety  changes  and  deteriorates  by  the,  segregation 
and  recombination  of  the  numerous  elementary  species 
that  compose  it. 

By  artificial  self-pollination  for  several  generations  (e.g., 
(five  or  more)  less  complex  strains  result,  which  are  homo- 
zygous  for  one  or  more  characters,  and  the  yield  per  acre 
may  thus  become  greatly  reduced.2  If  now,  two  of  these 
simplified  strains,  homozygous  for  many  characters,  and 

1  If  both  parents  have  blue  eyes  the  children  with  rare  exceptions  have 
blue  eyes;  if  one  parent  has  brown  eyes  and  one  blue,  the  children  may  be 
both  blue-  and  brown-eyed,  or  all  brown-eyed,  for  brown  eye-color  tends 
to  be  dominant  over  blue  color.  When  both  parents  have  brown  eyes, 
part  of  the  children  may  have  blue  eyes  and  part  of  them  brown,  or  they 
may  all  be  brown-eyed.  As  usetl  here,  the  term  "brown-eyes"  means  all 
eyes  having  brown  pigment,  whether  in  small  spots  (gray  eyes),  or  traces 
(hazel  eyes),  or  generally  distributed  (brown,  or  sometimes  black,  eyes).. 
The  term  "blue  eyes"  designates  only  those  cases  in  which  brown  pig- 
ment is  entirely  lacking. 

2If  a  high-yielding  strain  was  separated  out  by  selection,  the  yield 
would  of  course  be  increased  above  the  average  of  the  mixed  field. 


70  HEREDITY   AND    EVOLUTION   IN   PLANTS 


EXPERIMENTAL   STUDY   OF   HEREDITY  71 

having  a  low  yield  per  acre,  are  crossed,  there  results  an 
FI  hybrid  progeny  that  is  heterozygous  for  all  of  these 
characters.  This  heterozygosity  is  correlated  with  a 
greatly  increased  vigor;  the  plants  are  much  larger,  and 
the  yield  per  acre  is  enormously  increased  (Fig.  43). 
Thus  in  one  experiment  of  this  kind  the  average  yield  of 
the  heterozygous  horticultural  variety  was  61.25  bushels 
per  acre.  After  self-fertilization  for  several  generations 
the  yield  became  reduced  to  29.04  bushels  per  acre;  but 
in  the  FI  generation  of  a  cross  between  two  of  these  self- 
fertilized  strains  the  yield  per  acre  rose  at  once  to  68.07 
bushels.  In  the  F2  generation  the  yield  again  fell  to  44.62 
bushels.  From  this,  and  numerous  other  experiments,  it 
is  found  that  the  biggest  corn  crop  is  to  be  obtained  by 
finding  the  strains  that  will  produce  the  largest  yield 
when  crossed,  and  then  using  for  seed  the  grains  of  the 
first-generation  hybrids  each  year. 

62.  Breeding  for  Disease -resistance. — Biffen,  in 
England,  crossed  a  wheat  of  poor  quality,  but  resistant 
to  rust  disease  (Puccinia  glumarum),  with  a  superior 
variety  but  very  susceptible  to  the  disease.  Suscepti- 
bility proved  dominant  in  the  FI  generation,  but  in  the 
F2  generation  disease-resistant  forms  appeared,  of  superior 

FIG.  43. — Zea  Mays.  In  the  experiment,  the  results  of  which  are  here 
illustrated,  nine  strains  of  Indian  corn  were  selected  according  to  the 
number  of  rows  of  kernels  on  the  cob,  varying  from  8  to  24  rows.  These 
were  pollinated  by  hand  each  year,  with  mixed  pollen,  in  such  manner  that 
self-pollination  was  entirely  prevented.  An  average  ear  of  each  strain  is 
shown  in  the  first  row  above.  In  the  second  row  is  shown  an  average 
ear  of  each  strain  after  self-fertilization  for  five  generations.  Note  the 
resulting  decrease  in  the  number  of  rows,  lack  of  filling  out  of  the  ears, 
and  other  marks  of  inferiority.  The  last  row  shows  the  remarkable  and 
immediate  increase  of  vigor  resulting  in  the  F\  generation  of  hybrids  be- 
tween various  pairs  of  the  selfed  strains.  (Photo  supplied  by  G.  H.  Shull.) 


72  HEREDITY   AND    EVOLUTION   IN   PLANTS 

quality,  which  bred  true  for  resistance.  The  water- 
melon, in  the  southern  states,  is  subject  to  a  very  de- 
structive disease  which  causes  a  wilting  of  the  vines  "and 
consequent  loss  of  fruit.  By  crossing  the  ordinary  non- 
resistant  watermelon  with  the  closely  related  common 
citron,  which  is  wilt  resistant,  W.  A.  Orton,  of  the  United 
States  Department  of  Agriculture,  produced  a  water- 
melon resistant  to  this  disease.  Numerous  other  illus- 
trations might  be  given.  This  is  becoming  one  of  the 
common  and  successful  methods  of  combating  plant 
disease. 

63.  Unsolved  Problems* — Like  all  truly  great  con- 
tributions to  science,  Mendel's  discoveries  have  raised 
more  questions  than  they  have  answered.  Therein  lies, 
in  part,  their  great  value.  So,  also,  the  most  important 
effect  of  Darwin's  work  was  that  it  set  men  to  asking 
questions.  The  history  of  botany,  as  of  all  natural  science 
since  1859,  is  chiefly  the  attempts  of  men  to  answer  the 
questions  raised  by  Darwin,  or  stimulated  in  their  own 
minds  by  his  books.  So  with  Mendel  and  de  Vries; 
biological  science,  since  1900,  has  been  largely  occupied 
in  trying  to  answer  the  questions  raised  by  these  men. 

What  are  these  questions?  There  is  not  space  here 
even  to  ask  them  all,  much  less  to  endeavor  to  answer 
them  even  briefly;  but  they  include  the  following  large 
problems: 

i.  Are  acquired  characters  inherited?  In  other  words, 
do  characteristics  acquired  after  birth  by  the  body  or 
mind  of  the  parent,  either  by  its  own  activity  or  as  a  re- 
sult of  the  immediate  effects  of  environment,  influence 
the  germ-cells  so  as  to  alter  the  inheritance  which  they 
transmit?  Some  say  yes,  others  say  no;  others  say,  only 


EXPERIMENTAL   STUDY   OF   HEREDITY  73 

in  part.  There  seems  to  be  evidence  both  ways,  but  the 
bulk  of  the  evidence  and  the  weight  of  scientific  opinion 
is  against  the  inheritance  of  acquired  characters  as  here 
denned.  We  can  arrive  at  the  correct  answer  only  by  care- 
ful experimentation,  that  is,  by  asking  questions  of  nature. 1 

2.  Can  the  inheritance  of  a  strain  be  artificially  altered? 
This  is  a  question  of  the  very  first  importance.     If  the 
inheritance  could  be  so  altered  the  marvels  that  breeders 
might  perform  would  be  greatly  increased.     A  blue  rose 
(the  despair  of  all  plant  breeders)  might  possibly  be  pro- 
duced by  sufficiently  careful  and  extended  experiment- 
ing; disease  and  undesirable  traits  of  character  might  be 
eliminated  from  certain  tainted  family  strains  by  artificial 
treatment.     On  the  other  hand,  by  an  unfortunate  com- 
bination   of    circumstances,    most    undesirable    and    re- 
grettable results  (e.g.,  a  weed  poisonous  to  cattle,  or  a  new 
and  virulent  disease-causing  bacterium)  might  be  experi- 
mentally produced.     The  experiment  has  been  made  of 
exposing  the  ovaries  of  flowers  to  the  rays  of  radium,  and 
of  injecting  them  with  various  chemical  substances,  with 
an  idea  of  altering  the  physical  or  chemical  nature  of  the 
egg-cells,  and  thus  altering  the  inheritance.     The  results 
of  such  experiments,  so  far  tried,  need  to  be  further  con- 
firmed before  we  can  say  with  certainty  that  the  result 
sought  has  been  accomplished. 

3.  How  may  dominance  be  explained?     Why  is  tallness 
dominant   over   dwarfness,  brown  eye-color  over  blue, 
any  one  character  over  any  other?     At  present  we  can 
only  speculate  on  these  questions.  , 

4.  What  is   the  mechanism   of  inheritance?     In  other 
words,  by  what  arrangement  and  interaction  of  atoms 

'On  the  inheritance  of   acquired   characters,    see  Thomson,  J.   A., 
Heredity.    London,  1908.     Chapter  VII. 


74  HEREDITY   AND   EVOLUTION   IN   PLANTS 

and  molecules  is  it  made  possible  that  the  peculiar  tone 
of  one's  voice,  the  color  of  a  rose,  the  odor  of  a  carnation, 
the  evenness  (or  otherwise)  of  one's  disposition,  may  be 
transmitted  from  one  generation  to  another?  How  may 
it  be  transmitted  through  one  generation,  without  causing 
any  external  expression,  and  reappear  in  the  second  gen- 
eration removed?  Is  the  cytoplasm  the  carrier,  or  the 
chromatin,  or  both  combined,  or  neither?  Is  the  transfer 
accomplished  by  little  particles  (pangens},  as  de  Vries 
contends,  or  by  chondriosomes,  or  otherwise?  We  do 
not  definitely  know,  but  many  careful  investigations  point 
to  the  chromatin  as  the  bearer  of  the  hereditary  factors. 
64.  Weismannism. — It  was  a  botanist,  Nageli,  who  first 
recognized  and  clearly  stated  that  inheritance  must  depend 
upon  a  least  quantity  of  matter,  and  numerous  experi- 
ments by  both  botanists  and  zoologists  soon  made  it 
evident  that  the  hereditary  substance  is  in  the  cell- 
nucleus,  rather  than  in  the  cytoplasm  surrounding  the 
nucleus.  Nageli  called  the  hereditary  substance  idio- 
plasm. Observations  of  the  germ-cells  of  plants  by  Stras- 
burger,  and  of  the  germ-cells  of  animals  by  O.  Hertwig, 
led  them  to  conclude  that  the  chromosomes  of  the  dividing 
nucleus  (Fig.  30  )are  the  locus  of  the  hereditary  substance. 
The  subsequent  evidence  upon  which  this  conclusion  rests 
is  too  voluminous,  and  some  of  it  too  technical,  to  be  pre- 
sented here  in  any  detail.  *  As  an  illustration  there  may 
be  cited  the  experiment  of  Boveri  who  removed  the 
nucleus  from  the  egg-cell  of  one  species  of  sea-urchin,  and 
then  caused  the  remaining  cytoplasm  to  be  fertilized  with 
a  sperm-cell  of  another  species  of  sea-urchin;  the  result- 
ing larva  possessed  only  paternal  characters. 

1  See  Morgan,  T.  H.     The   physical  basis  of  heredity.      Philadelphia, 
1919. 


EXPERIMENTAL   STUDY   OF   HEREDITY  75 

Weismann  expanded  the  above  conception  of  hereditary 
substance  by  calling  attention  to  the  fact  that  it  must 
contain  elements,  not  only  from  one  individual  or  pair, 
but  from  a  long  line  of  ancestors.  He  called  the  idioplasm 
(of  Nageli)  in  the  germ-cells  germ-plasm,  and  the  heredi- 
tary units,  "necessary  to  the  production  of  a  complete 
individual,"  he  called  ids.  Each  id  contains  a  full  com- 
plement of  hereditary  factors  necessary  to  produce  a 
perfect  plant  or  animal.  The  germ-plasm  corresponds 
to  the  deeply  staining  chromatin  of  the  cell-nucleus,  and 
the  ids  are  grouped  together  in  idants,  which  correspond, 
in  general,  to  the  chromosomes.  Weismann  further 
postulated  that  the  ids  were  composed  of  "primary  con- 
stitutents,"  which  he  called  determinants,  and  that  every 
character  independently  inherited  has  its  own  determinant 
in  the  germ-plasm.  Finally  Weismann  postulated  that 
each  determinant  is  a  complex  of  biophors  (the  ultimate 
units  of  matter  in  the  living  state),  each  biophore  being 
composed  of  (non-living)  chemical  molecules.  Thus  we 
rise  through  his  categories  as  follows,  from  atom  to  mole- 
cule, from  molecule  to  biophore,  from  biophore  to  deter- 
minant, from  determinant  to  ids,  from  ids  to  idants 
(chromosomes),  which  are  composed  of  the  hereditary 
substance  or  germ-plasm;  schematically  as  follows: 

germ-plasm  (chromatin) 
idant  (chromosome) 
id. 

determinant  (factor,  of  Mendel) 
biophore  (biogen,  of  Verworn) 
molecule 
atom 

The  germ-plasm  is  continuous  from  generation  to  genera- 
tion, and  therefore  possesses  a  kind  of  physical  immortality. 


76  HEREDITY   AND   EVOLUTION   IN   PLANTS 

65.  Relation  of  Weismannism  to  Mendelism. — It  will 
readily  be  recognized  that  the  "determinants"  of  Weis- 
mann    are   the    "factors"    of   Mendelian   nomenclature. 
Morever,    it   follows  logically  from  Weismann's  theory 
that  acquired  characters  are  not  inherited,  an  inference 
that  agrees  with  observation  and  experiment.     Nageli, 
director  of  the  botanic  garden  in  Munich,  transplanted 
specimens  of  Hawkweed  (Hieraceum)  from  the  high  Alps 
to  the  lower  altitude  and  changed  climate  of  his  garden, 
and  these  plants  began  to  manifest  new  characters  which 
reappeared  in  successive  generations  for  more  than  a 
decade.     This   looked  like  the  inheritance   of   acquired 
characters,  but  when  the  plants  were  subsequently  taken 
back  to  the  high  Alps,  they  failed  to  manifest  the  charac- 
ters expressed  in  the  botanic  garden,  reverting  to  their 
former  alpine  characteristics.     Thus  it  is  seen  that  the 
reappearance  of  the  new  characters  in  successive  genera- 
tions in  the  botanic  garden  was  not  due  to  inheritance  of 
these  acquired  characters,  but  to  the  continuity  of  the 
new  environment.     The  inheritance  had  not  been  altered 
though  the  expression  of  it  had.     This  is  in  agreement  with 
what  we  should  expect  from  the  definition  of  inheritance 
given  on  page  so.1 

66.  Eugenics. — Students  of  biology  have  been  quick 
to  recognize  the  fact  that,  if  we  correctly  understand  the 
laws  of  heredity,  we  are  in  a  position  to  apply  them,  not 
only  to  plants  and  the  lower  animals,  but  to  mankind. 
The  application  of  the  laws  of  heredity  in  a  way  to  produce 
a  healthier  and  more  efficient  race  of  men  constitutes  the 
practice  of  eugenics. 2    The  underlying  principles  of  eugenics 

1  See  also  pages  48  and  66-67. 

2  The  word  eugenics  is  from  two  Greek  words  meaning  well  born. 


EXPERIMENTAL    STUDY   OF   HEREDITY  77 

are  of  course,  very  largely  those  of  heredity.  Eugenics  is 
the  applied  science  based  upon  the  pure  science  of  heredity. 
The  main  problem  of  eugenics  is  how  to  eliminate  human 
beings  with  a  tendency  to  any  physical  or  mental  weakness 
making  for  poverty,  misery,  ignorance,  and  crime;  and 
how  to  increase  the  number  of  individuals  physically, 
mentally,  and  morally  more  robust  and  sound;  and  withal 
how,  if  possible,  to  raise  the  standard  of  all  desirable 
human  traits.  A  careful  study  of  heredity  and  eugenics 
will  make  possible  a  much  more  intelligent  and  efficient 
program  for  charity  work  and  social  betterment. 

67.  Investigations  Since  Mendel. — It  must  be  re- 
membered that  Mendel's  most  valued  contribution  was 
not  the  observations  which  he  made  and  recorded  con- 
cerning the  garden  pea,  nor  the  hypotheses  which  he  ad- 
vanced on  the  basis  of  those  observations,  but  this  method 
of  procedure,  whereby  he  elevated  the  study  of  heredity 
to  the  rank  of  an  exact  science.  As  in  the  case  of  all 
hypotheses,  the  task  for  science  is  to  subject  them  to  -the 
most  searching  tests,  to  see  if  they  invariably  agree  with 
facts,  and  may  be  accepted  as  in  all  probability  embody- 
ing the  actual  truth — may  be  elevated  to  the  rank  of 
theories.  The  testing  of  Mendelism  has  been  occupying 
all  the  best  talents  of  many  investigators  since  the  re- 
discovery of  Mendel's  publication,  about  1900.  Many 
biologists  are  still  skeptical,  a  few  reject  the  hypotheses, 
and  still  others  believe  they  contain  the  germ  of  truth, 
but  must  be  more  or  less  modified.  Whether  they  prove 
to  be  erroneous  or  true  is  not  so  important,  but  it  is  impor- 
tant for  us  to  know  which  is  the  case.  True  or  not,  they, 
like  nearly  all  working  hypotheses  (natural  selec- 
tion, mutation,  nebular  hypothesis,  atomic  hypothesis 


78  HEREDITY   AND   EVOLUTION   IN   PLANTS 

in  chemistry,  etc.)  are  rendering,  or  have  rendered,  a 
priceless  service  to  science  by  pointing  the  way  to  further 
study,  which  enriches  our  knowledge  of  the  living  world, 
including  ourselves,  and  therefore  increases  the  intelli- 
gence with  which  we  may  order  our  own  conduct  and  lives. 
If  the  study  of  plants  had  rendered  no  other  service  to 
mankind  than  this  contribution  of  an  effective  method  of 
ascertaining  the  laws  of  heredity,  it  would  have  amply 
justified  all  the  ardubus  labor  that  men  have  devoted  to 

it  for  2,000  years.1   / 
/ 

1  Only  one  of  the  simplest  cases  worked  out  by  Mendel  is  summarized 
in  this  chapter.  A  more  thorough  study  of  his  experimental  results  and 
theories  must  be  reserved  for  more  advanced  study. 


CHAPTER  VI 
EVOLUTION     . 

68.  Doctrine  of  Special  Creation. — In  the   time   of 
Linnasus,  the  "father  of  botany,"  men  believed  that  the 
seven  "days"  of  creation  left  the  world  substantially  as 
we  now  find  it.     The  stars  and  planets,  mountains  and 
oceans,  plants  and  animals  were  created  once  and  for  all, 
and  continued  without  important  change  until  the  present. 
In  the  beginning,  as  now,  there  were  the  same  oceans  and 
hills,  the  same  kinds  of  plants,  and  the  same  kinds  of 
animals.     Nor,   it  was   believed,    are   any  fundamental 
changes  now  in  progress.     Creation  was  not  continuous; 
it  took  place  within  a  brief  period  (seven  "days"),  and 
then  ceased;  after  that  the  Creator  merely  watched  over 
the  objects  of  his  handiwork.     Opposed  to  this  doctrine 
is  the  theory  of  evolution. 

69.  Meaning  of  Evolution. — Evolution  means  gradual 
change.     Applied   to   the  natural  world  the  theory  of 
evolution  is  the  direct  opposite  of  the  doctrine  of  special 
creation.     It  teaches  that  things  were  not  in  the  beginning 
as  we  now  find  them,  but  that  there  has  been  constant 
though  gradual  change.     Creation  is  regarded,  not  as 
having  taken  place  once  and  for  all,  but  as  being  a  con- 
tinuous process,  operating  from  the  beginning  without 
ceasing — and  still  in  progress. 

70.  The   Course   of  Evolution. — The   theory   teaches 
that   the   gradual   changes   have   been   from   relatively 
simple  conditions  to  those  more  complex.     The  compli- 

79 


8o  HEREDITY   AND   EVOLUTION   IN   PLANTS 

cation  has  been  two-fold:  (i)  simple  individuals,  whether 
mountains,  rivers,  planets,  animals,  or  plants,  have  become 
more  complex  (e.g.,  compare  the  structure  of  the  plant, 
Pleurococcus,  a  simple  spherical  cell,  with  that  of  the  fern) ; 

I  (2)  the  relation  between  living  things,  and  between  them 
and  their  surroundings  has  become  more  complex  (e.g., 
compare  a  unicellular  bacterium,  with  its  relatively  simple 
life  relations,  with  the  clover  plant,  highly  organized,  and 
related  to  water,  air,  soil,  light,  temperature,  gravity, 

|  bacteria  (in  its  roots),  and  insects  (for  cross-pollination)). 

Most  of  the  steps  of  evolution  have  been  progressive, 

toward  higher  organization,  greater  perfection  of  parts, 

|  increased  efficiency  of  function,  as,  for  example,  from 
algae  having  one  or  a  few  cells  only,  to  flowering  plants, 
like  roses  and  orchids;  but  not  all  the  steps  have  been  in 
this  direction.  Some  of  the  steps  have  been  regressive, 
toward  simpler  organization,  less  perfection  of  parts, 
decreased  efficiency  of  function,  as,  for  example,  from 
green  algae  to  the  non-green,  alga-like  fungi  (Phycomy- 
cetes,  such  as  bread  mold),  from  independence  to  parasitism 
(mistletoe  and  dodder),  or  to  saprophytism  (Indian  pipe 
and  toad-stools). 

The  thirty  odd  species  of  the  Duckweed  family,  related 
to  the  Arum  family  (Jack-in-the-pulpit,  calla,  skunk  cab- 
bage, sweet  flag,  etc.),  illustrate  regression;  they  comprise 
the  simplest,  and  some  of  them  the  smallest  of  all  flowering 
plants.  The  plant  body  of  Lemna  is  a  tiny  disc-shaped, 
thallus,  having  a  central  vein  (vascular  strand)  with  or 
without  branches.  Each  plant  has  one  root  with  no 
vascular  tissue.  The  flowers,  borne  on  the  margin  or 
upper  surface  of  the  thallus,  have  one  simple  pistil  and 
only  one  stamen  (Fig.  44).  The  dozen  or  more  species 


EVOLUTION 


8 1 


of  Wolffia  possess  still  simpler  bodies,  somewhat  globose, 
with  neither  roots,  veins,  nor  other  organs,  except  flowers; 
even  flowers  are  unknown  in  some  species  (e.g.,  Wolffia  i 
populifera,  Fig.  44).  Wolffia  punctata  measures  only 
0.5-0.8  mm.  long.  The  plants  are  fittingly  described  in 
the  manuals  as  "minute,  alga-like  grains,"  floating  on  or 


FIG.  44- — Lemnaceae.  a,  b,  c,  Lcmna  trisidca;  d,  Wolffia  punctata; 
e,  f,  Wolffia  papulifera.  (Redrawn  from  Britton  and  Brown,  slightly 
modified.) 

just  beneath  the  surface  of  still  water.  Some  botanists 
consider  the  plant  body  as  morphologically  a  frond,  others 
as  a  leafless  stem.  Since  the  first  plant-body  from  the  seed 
is  only  a  matured  cotyledon,  or  seed-leaf,  Goebel  considers 
that  it  cannot  be  interpreted  as  other  than  a  free-living 
leaf.  These  tiny,  simple  plants  are  considered  to  have 


82  IIKREDITY   AND    EVOLUTION   IN   PLANTS 

originated  by  regressive  evolution,  their  simplification 
being  closely  correlated  with  a  reversion  from  dry  land  to 
an  aquatic  habit  of  life.  A  similar  reduction  of  structure 
is  found  in  the  tiny  floating  ferns,  Sakinia  and  Azolla. 

71.  Inorganic  Evolution. — The  process  of  evolution  is 
not  confined  to  living  things,  but,  as  indicated  above, 
applies  to  all  nature.  Even  the  chemical  elements  are 
believed  to  have  been  produced  by  evolutionary  changes, 
and  to  be  even  now  in  process  of  evolution.  This  is  one 
of  the  results  of  the  recently  discovered  phenomenon  of 
radioactivity,  which  is  essentially  the  transformation  of 
the  atoms  of  one  chemical  element  into  those  of  another. 
Fossil  remains  of  marine  animals  and  plants,  found  im- 
bedded in  the  rocks  on  mountain  summits,  indicate,  with- 
out possibility  of  reasonable  doubt,  that  what  is  now 
mountain  top  was  formerly  ocean  bottom.  The  mountain 
has  come  to  be,  by  a  series  of  gradual  changes.  Rivers 
and  valleys  are  constantly  changing  so  that  the  present 
landscape  is  the  result  of  evolutionary  processes;  climates 
have  changed,  as  we  know  from  the  fact  that  fossil  re- 
mains of  tropical  plants  are  now  found  in  the  rocks  in 
arctic  regions;  the  atmosphere  and  the  water  of  the  ocean 
have  reached  their  present  condition  as  the  result  of  gradual 
transformations  extending  over  aeons  of  time;  even  the 
stars  and  planets,  like  our  own  earth,  are  coming  gradually 
into  being,  undergoing  changes  of  surface  and  interior 
condition,  and  ceasing  to  exist.  Nothing  is  constant  except 
constant  change.  The  main  problem  of  astronomy  is  to 
ascertain  and  record,  in  order,  the  evolutionary  changes 
that  have  resulted  in  the  present  system  of  suns  and 
planets.  The  main  problem  of  geology  is  to  ascertain  and 
record,  in  order,  the  evolutionary  steps  that  have  resulted 
in  the  present  condition  of  the  earth. 


EVOLUTION  83 

72.  Fitness  of  the  Environment. — Biological  literature 
has  always  taken  account  of  what  has  been  called  "adapta- 
tion," or  the  fitness  of  living  things  for  life  in  the  surround- 
ings or  environment  where  they  are  placed.     But  a  recent 
writer,1    has    elaborated    the    complimentary    notion,  of 
the  fitness  of  the  environment.     Recognizing  living  things  as 
"  mechanisms  which  must  be  complex,  highly  regulated, 
and  provided  with  suitable  matter  and  energy  as  food," 
he  shows  that  the  present  inorganic  environment  is  the 
best  conceivable.     Inorganic  evolution  has  resulted,  among 
other  things,  in  the  occurrence  of  large  quantities  of  water 
and  carbon  dioxide;  their  physical  and  chemical  properties, 
and  those  of  the  ocean,  together  with  the  chemical  properties 
of  the  elements,  carbon,  oxygen,  and  hydrogen,  and  their 
numerous  compounds,  "are  in  character  or  in  magnitude 
either  unique  or  nearly  so,  and  are  in  their  effect  favorable ' ' 
to  the  organisms  with  which  we  are  familiar,  and  which 
possess  the  three  fundamental  characteristics  of  complexity, 
regulation,  and  metabolism.     The  elements  carbon,  hydro- 
gen, and  oxygen,  says  Henderson,  are  uniquely  and  most 
highly  fitted  to  be  the  stuff  of  which  life  is  formed,  and  of 
the  environment  in  which  it  exists. 

73.  Organic    Evolution. — Developmental    changes    in 
living  things  constitute  organic  evolution.     Such  changes 
are  manifested  in  the  development  of  an  individual  from 
a  spore  or  an  egg.     The  development  of  a  mature  in- 
dividual is  ontogeny.     The  development  of  a  group  of 
related  forms  (genera,  families,  orders,  etc.)  is  phylogeny. 
The  chief  problem  of  biology  is  to  ascertain  and  record, 
in  order,  the  evolutionary  changes  that  have  resulted  in 

1  Henderson,    Lawrence    J.     The    fitness  of   the   environment.     New 
York,  1913. 


84  HEREDITY   AND   EVOLUTION   IN   PLANTS 

the  appearance  of  life  and  the  present  condition  of  living 
things. 

The  major  problem  of  botany  is  to  record,  in  order,  the 
evolutionary  steps  that  have  culminated  in  the  present  con- 
dition of  the  plant  world. 

Organic  evolution  means  that,  after  the  first  appearance 
of  life,  all  living  things,  plant  or  animal,  have  been 
derived  from  preexisting  living  things,  in  other  words,  that 
the  present  method  of  formation  of  living  things,  by  the 
reproduction  of  organisms  already  existing,  has  always 
been  the  method — "Omne  vivum  ex  ovo"  (all  life  from  an 
egg),  "omne  vivum  e  vivo"  (all  life  from  preexisting  life). 

74.  Method  of  Evolution. — To  recognize  that  evolution 
is  the  method  of  creation  still  leaves  unanswered  the  im- 
portant question  as  to  the  method  of  evolution.  By  what 
process  was  the  gradual  development  of  the  living  world 
accomplished?  Various  hypotheses  have  been  elaborated 
in  answer  to  this  question.  We  can  here  only  briefly 
outline  three  of  the  most  important  ones. 

i.  Agassiz's  Hypothesis. — The  great  teacher  and  student 
of  nature,  Louis  Agassiz,  believed  that  the  vast  array  of 
plant  and  animal  species,  past  and  present,  had  no  material 
connection,  but  only  a  mental  one;  that  is,  they  merely  re- 
flected the  succession  of  ideas  as  they  developed  in  the 
mind  of  the  Creator,  but  were  not  genetically  related  to 
each  other.  "We  must  .  .  .  look  to  some  cause  outside 
of  Nature,  corresponding  in  kind  to  the  intelligence  of 
man,  though  so  different  in  degree,  for  all  the  phenomena 
connected  with  the  existence  of  animals  in  their  wild 
state.  .  .  .  Breeds  among  animals  are  the  work  of  man: 
Species  were  created  by  God."1 

1  Agassiz,  L.  "Methods  of  Study  in  Natural  History,"  Boston,  1893, 
pp.  146,  147. 


EVOLUTION 


But  to  state  that  species  were  created  by  God  does  not 
satisfy  the  legitimate  curiosity  of  the  scientific  man. 
What  he  wishes  to  know  is :  By  what  method  was  creation 
accomplished?  God  might  have  worked  in  various  ways. 
Now,  the  study  of  Nature  has  never  revealed  to  us  but  one 
method  by  which  living  things  originate,  and  that  is  by 
descent  from  preexisting  parents.  Agassiz's  hypothesis 


FIG.  45. — Louis  Agassiz.     (From  Ballard's  " Three  Kingdom.") 

contradicts  this.  All  oaks  now-a-days  are  derived  by 
descent  from  preexisting  oaks,  but  the  first  oak,  accord- 
ing to  the  doctrine  of  special  creation,  was  created  by 
supernatural  means ;  it  had  no  ancestors.  The  chief  objec- 
tion to  the  acceptance  of  this  hypothesis  is  that  the  more 
profoundly  and  accurately  we  study  living  things,  the  more 
obvious  it  becomes  that  truth  lies  in  another  direction. 


86 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


2.  Lamarck's  Hypothesis. — The  noted  French  naturalist, 
Lamarck,  taught  that  all  living  things  have  been  derived 
from  preexisting  forms;  that  the  effects  of  use  and  disuse 
caused  changes  in  bodily  structure;  that  these  changes 
were  inherited  and  accentuated  from  generation  to  genera- 
tion; that,  being  of  use,  those  individuals  possessing  the 
changes  in  greatest  perfection  survived,  while  others  per- 


FIG.  46. — Water  buttercup  (Ranunculus  aquatilis) ,  showing  aerial 
leaves  (a),  and  aquatic  leaves  (w).  f,  fruit.  Drawn  from  herbarium 
specimen. 

ished;  and  that  the  derivation  of  new  species  is  thus  ac- 
counted for  in  a  simple  and  logical  manner.  By  continual 
reaching  for  tender  leaves  on  high  branches,  the  long  neck 
of  the  giraffe  was  gradually  produced,  the  slight  gain  in 
length  in  one  generation  being  transmitted  by  inheritance 
to  the  next,  and  so  on. 

The  main  thesis  of  Lamarck,  as  stated  by  himself,  is 
as  follows : 


EVOLUTION  87 

"In  animals  and  plants,  whenever  the  conditions  of 
habitat,  exposure,  climate,  nutrition,  mode  of  life,  et  cetera, 
are  modified,  the  characters  of  size,  shape,  relations  be- 
tween parts,  coloration,  consistency,  and,  in  animals, 
agility  and  industry,  are  modified  proportionately." 

As  illustrating  the  direct  effect  of  environment  on  organ- 
isms, Lamarck  chose  a  plant,  the  water-buttercup  (Ran- 
unculus aquatilis),  which  may  grow  in  marshy  places,  or  im- 
mersed in  water  (Fig.  46).  When  immersed,  the  leaves 
are  all  finely  divided,  but  when  not  immersed,  they  are 
merely  lobed. 

While  plants  are  more  passive,  and  are  affected  by  their 
surroundings  directly,  through  changes  in  nutrition,  light, 
gravity,  and  so  on,  animals  react  to  environmental  changes 
in  a  more  positive  and  less  passive  manner.  Thus,  in 
the  words  of  Lamarck:1 

"important  changes  in  conditions  bring  about  impor- 
tant changes  in  the  animals'  needs,  and  changes  in  their 
needs  bring  about  changes  in  their  actions.  If  the  new 
needs  become  constant  or  durable,  the  animals  acquire 
new  habits.  .  .  .  Whenever  new  conditions,  becoming 
constant,  impart  new  habits  to  a  race  of  animals  .  .  . 
these  habitual  actions  lead  to  the  use  of  a  certain  part  in 
preference  to  another,  or  to  the  total  disuse  of  a  part  which 
is  now  useless ....  The  lack  of  use  of  an  organ,  made 
constant  by  acquired  habits,  weakens  it  gradually  until 
it  degenerates  or  even  disappears  entirely."  Thus,  "it 
is  part  of  the  plan  of  organization  of  reptiles,  as  well  as  of 
other  vertebrates,  that  they  have  four  legs  attached  to 
their  skeleton  .  .  .  but  snakes  acquired  the  habit  of  glid- 

1  Translated  from  his  Philosophic  Zoologique,  vol.  I,  pp.  227,  223,  224, 


HEREDITY   AND    EVOLUTION   IN   PLANTS 

ing  over  the  ground  and  concealing  themselves  in  the  grass ; 
owing  to  their  repeated  efforts  to  elongate  themselves,  in 
order  to  pass  through  narrow  spaces,  their  bodies  have 
acquired  a  considerable  length,  not  commensurate  with 
their  width.  Under  the  circumstances,  legs  would  serve 
no  purpose  and,  consequently,  would  not  be  used,  long 
legs  would  interfere  with  the  snakes'  desire  for  gliding, 
and  short  ones  could  not  move  their  bodies,  for  they  can 
only  have  four  of  them.  Continued  lack  of  use  of  the 
legs  in  snakes  caused  them  to  disappear,  although  they 
were  really  included  in  the  plan  of  organization  of  those 
animals." 

On  the  other  hand,  "the  frequent  use  of  an  organ,  made 
constant  by  habit,  increases  the  faculties  of  that  organ, 
develops  it  and  causes  it  to  acquire  a  size  and  strength  it 
does  not  possess  in  animals  which  exercise  less.  A  bird, 
driven  through  want  to  water,  to  find  the  prey  on  which 
it  feeds,  will  separate  its  toes  whenever  it  strikes  the  water 
or  wishes  to  displace  itself  on  its  surface.  The  skin  uniting 
the  bases  of  the  toes  acquires,  through  the  repeated  sepa- 
rating of  the  toes,  the  habit  of  stretching;  and  in  this  way 
the  broad  membrane  between  the  toes  of  ducks  and  geese 
has  acquired  the  appearance  we  observe  to-day." 

If  such  modifications  are  acquired  by  both  sexes  they 
are  transmitted  by  heredity  from  generation  to  generation. 
This  hypothesis  is  known  as  "the  inheritance  of  acquired 
characters." 

One  of  the  weaknesses  in  Lamarck's  hypothesis  appears 
in  his  illustration  of  the  snake.  If  we  should  grant  that 
inheritance  of  the  effects  of  disuse  of  the  legs  might  possi- 
bly explain  their  absence  in  snakes,  still  it  would  not  ex- 
plain the  origin  of  the  snake's  desire  to  glide.  That  is,  of 


EVOLUTION  89 

course,  as  much  a  characteristic  of  the  snake  as  the  absence 
of  legs. 

Other  arguments  against  the  validity  of  Lamarckism 
are:  first,  that  no  one  has  ever  been  able  to  prove,  by  ex- 
periment or  otherwise,  that  the  effects  of  use  (the  so-called 


FIG.  47. — Jean  Baptiste  Lamarck  (1744-1829).  He  elaborated  the 
hypothesis  of  organic  evolution  by  inheritance  of  the  effects  of  use 
and  disuse. 

" acquired  characters")  are  inheritable,  while  innumerable 
facts  indicate  that  they  are  not;  second,  the  hypothesis 
could  apply  only  to  the  animal  kingdom,  since  plants  in 
general  have  no  nervous  and  muscular  activities  like  those 
of  animals.  A  hypothesis  of  organic  evolution,  to  be  valid, 
must  apply  equally  to  both  plants  and  animals. 

3.  Darwin's  Hypothesis. — This  will  be  outlined  in  the 
next  chapter. 


CHAPTER  VII 
DARWINISM 

75.  Darwin  and  Wallace. — The  question  of  the 
method  of  evolution  continued  to  be  debated,  with  no 
satisfactory  solution  in  sight,  until  1859, *  when  Charles 
Darwin  published  the  greatest  book  of  the  nineteenth 
century,  and  one  of  the  greatest  in  the  world's  history, 
the  Origin  of  Species.'2  This  book  was  the  result  of  over  20 
years  of  careful  observation  and  thought.  It  consisted 
of  the  elaboration  of  two  principal  theories:  (i)  that 
evolution  is  the  method  of  creation;  (2)  that  natural 
selection  is  the  method  of  evolution. 

By  a  strange  coincidence  Alfred  Russell  Wallace,  also 
by  many  years  of  thoughtful  observation,  reading,  and 
reflection,  had  independently  formulated  the  conception 
of  natural  selection  in  far-off  Ternate,  and  embodied 
his  ideas  in  a  paper  which  he  sent  to  Darwin  for  the  purpose 
of  having  it  read  before  the  Royal  Society.  The  paper, 
with  its  accompanying  letter,  reached  Darwin  on  June 
18,  1858,  while  the  latter  was  engaged  in  writing  out 
his  own  views  on  a  scale  three  or  four  times  as  extensive  as 
that  afterward  followed  in  the  Origin  of  Species.  As  a 
result  of  the  unsurpassed  magnanimity  of  the  two  men, 
and  their  generous  attitude  toward  each  other,  it  was 

1  This  date  should  be  memorized.  It  is  one  of  the  most  important 
in  the  whole  history  of  human  thought. 

z  The  full  title  of  the  book  was  "The  Origin  of  Species  by  Natural 
Selection,  or  the  Preservation  of  Favored  Races  in  the  Struggle  for 
Life." 

90 


DARWINISM  91 

arranged  to  have  a  joint  paper  by  Darwin  and  Wallace 
presented  to  the  Society.  This  paper,  entitled  "On 
the  tendency  of  species  to  form  varieties;  and  on  the  per- 
petuation of  varieties  and  species  by  natural  means  of 
selection,"  was  presented  at  a  special  meeting  of  the  Society 


FIG.  48. — Charles  Darwin.  The  publication  of  his  "Origin  of  Species,' 
in  1859,  revolutionized  human  thought,  and  gave  direction  to  all  scientific 
and  philosophic  thinking  from  that  time  to  the  present. 

on  July  i,  1858,  being  read  by  the  secretary  in  the  absence 
of  both  Darwin  and  (of  course)  Wallace. 

76.  Early  Antagonism  to  Evolution. — The  concep- 
tion that  evolution  (as  distinguished  from  .periodic,  super- 
natural interventions  of  the  Deity)  is  the  method  of 


9 2  HEREDITY   AND    EVOLUTION   IN   PLANTS 

creation  was  arrived  at  independently  by  Darwin,  but  was 
not  new  with  him.  As  we  have  just  seen,  it  was  proposed 
by  Lamarck.  Greek  philosophers  2,000  years  previously 
had  suggested  the  idea;  but  it  had  never  won  the  general 
acceptance  of  the  educated  world,  partly  because  it  was 
feared  to  be  anti-religious,  partly  because  it  was  never 
substantiated  by  sufficiently  convincing  evidence,  and 
partly  because  of  the  antagonism  of  a  few  men  of  great 
influence  in  the  world  of  intellect.  Men  preferred  to  fol- 
low a  leader,  more  or  less  blindly,  rather  than  take  the 
pains  to  examine  the  voluminous  evidence  for  themselves, 
and  accept  the  logical  conclusion  without  prejudice  or 
fear,  wherever  it  might  lead  them,  or  however  much  it 
might  contradict  all  their  prejudice  and  preconceived 
notions.  But  truth  will  always,  in  the  end,  command 
recognition  and  acceptance,  and  there  is  now  almost  no 
scientific  man  who  does  not  regard  evolution  as  axiomatic. 
It  is  one  of  the  most  basic  of  all  conceptions,  not  only  in 
the  natural  and  the  physical  sciences,  but  also  in  history, 
sociology,  philosophy,  and  religion;  it  has,  indeed  com- 
pletely revolutionized  every  department  of  human 
thought. 

77.  Darwinism. — It  is  the  second  of  the  above  men- 
tioned theories,  i.e.,  natural  selection,  that  constitutes  the 
essence  of  Darwinism.  The  theory  is  based  upon  five 
fundamental  facts,  which  are  matters  of  observation,  and 
may  be  verified  by  anyone,  as  follows : 

i.  Inheritance. — Characteristics  possessed  by  parents 
tend  to  reappear  in  the  next  or  in  succeeding  generations. 
We  are  all  familiar  with  the  fact  that  children  commonly 
resemble  one  or  both  parents,  or  a  grandparent  or  great 
grandparent,  in  some  characteristic.  From  this  we  infer 


DARWINISM  93 

that  something  has  been  inherited  from  the  ancestor  which 
causes  resemblance  in  one  or  more  characters — physical  or 
mental. 

2.  Variation. — But  the  expression  of  the  inheritance  is 
seldom,  if  ever,  perfect.     Eyes  are  a  little  less  or  a  little 
more  brown;  stature  is  never  just  the  same;  one-half  the 
face  may  resemble  a  given  ancestor  more  than  another; 
petals  may  be  more  or  less  red  or  blue;  no  two  oranges 
taste  exactly  alike;  no  two  maple  leaves  are  of  precisely 
the  same  shape.     There  is  inheritance,  but  inheritance  is 
usually  expressed  with  modifications  or  variations  of  the 
ancestral  type. 

3.  Fitness  for  Environment. — It  is  common  knowledge 
that  living  things  must  be  adjusted  to  their  environment. 
Poor  adjustment  means  sickness  or  weakness;  complete 
or   nearly   complete   lack   of   adjustment   means   death. 
Water-lilies,    for    example,    cannot    live  in  the  desert, 
cacti  cannot  live  in  salt  marshes;  cocoanuts  cannot  be 
grown  except  in  subtropical  or  tropical  climates,  edelweiss 
will  not  grow  in  the  tropics.     This  is  because  these  various 
kinds  of  plants  are  so  organized  that  they  cannot  adjust 
themselves  to  external  conditions,  beyond  certain  more  or 
less  definite  limits  or  extremes.     A  cactus  is  fit  to  live  in 
the  desert  because  it  is  protected  by  its  structure  against 
excessive  loss  of  water,   and  has  special  provision  for 
storing  up  water  that  may  be  used  in  time  of  drought. 
Deciduous  tress  are  fitted  to  live  in  temperate  regions, 
partly  because  their  deciduous  habit  and  their  formation 
of  scaly  buds  enables  them  to  withstand  the  drought  of 
winter.     Negroes  live  without  discomfort  under  the  trop- 
ical sun  because  they  are  protected  by  the  black  pigment 
in  their  skin.     And  so,  in  countless  ways,  we  might  illus- 


94  HEREDITY   AND    EVOLUTION   IN   PLANTS 

trate  the  fact  that  all  living  things,  in  order  to  flourish, 
must  be  adjusted  to  their  surroundings. 

4.  Struggle  for  Existence.— The  clue  to  the  method  of 
evolution  first  dawned  upon  Darwin  in  1838,  while  reading 
Malthus  on  "Population."  Malthus  emphasized  the  fact 
that  the  number  of  human  beings  in  the  world  increased 
in  geometrical  ratio  (by  multiplication),  while  the  food  sup- 
ply increased  much  less  rapidly  by  arithmetical  ratio  (by 
addition).  Therefore,  argued  Malthus,  the  time  will  soon 
be  reached  when  there  will  not  be  food  enough  for  all; 
men  will  then  struggle  for  actual  existence,  and  only  the 
fittest  (i.e.,  the  strongest,  the  fleetest,  the  most  clever  or 
cunning)  will  survive.  In  pondering  this  hypothesis 
Darwin  at  once  saw  its  larger  application.1  There  are 
always  more  progeny  produced  by  a  plant  or  an  animal 
than  there  is  room  and  food  for,  should  they  all  survive. 
Darwin  showed  that  the  descendants  of  a  single  pair  of 
elephants  (one  of  the  slowest  breeders  of  all  animals) 
would,  if  all  that  were  born  survived,  reach  the  enormous 
number  of  19,000,000  in  from  740  to  750  years.2  But 
the  total  number  of  elephants  in  the  world  does  not  appre- 
ciably increase :  evidently  many  must  perish  for  every  one 
that  lives. 

iuln  October  1838,"  says  Darwin,  "that  is,  15  months  after  I  had 
begun  my  systematic  inquiry,  I  happened  to  read  for  amusement  'Malthus 
on  Population,'  and  being  well  prepared  to  appreciate  the  struggle  for 
existence  which  everywhere  goes  on  from  long-continued  observation  of 
the  habits  of  animals  and  plants,  it  at  once  struck  me  that  under  these 
circumstances  favorable  variations  would  tend  to  be  preserved,  and 
unfavorable  ones  to  be  destroyed.  The  result  of  this  would  be  the  forma- 
tion of  new  species.  Here  then  I  had  at  last  got  a  theory  by  which  to 
work." 

2  One  pair  of  elephants  produces  an  average  of  only  one  baby  elephant 
in  10  years,  and  the  breeding  period  is  confined  to  from  about  the  3oth  to 
the  goth  year. 


DARWINISM  95 

Linnaeus,  a  century  before  Darwin,  had  called  attention 
to  the  fact  that  if  an  annual  plant  produced  only  two  seeds 
a  year,  and  each  of  the  plants  from  these  seeds,  in  turn, 
produced  two  seeds  the  second  year,  and  so  on,  there  would, 
if  all  the  individuals  lived,  be  a  million  plants  at  the  end  of 
twenty  years.  But,  few  species  breed  as  slowly  as  that. 
According  to  Kerner,  the  common  broad-leaved  plantain 
(Plantago  major)  produces  14,000  seeds  annually;  shep- 
herd's purse  (Capsella  Bursa-pastoris),  64,  ooo ;  and  tobacco, 
360,000.  The  number  of  seeds  produced  each  year  by  the 
orchid,  Acropera,  was  carefully  estimated  by  Darwin  at 
74,000,000.  But  these  figures  are  wholly  surpassed  by 
the  ferns.  Professor  Bower  estimates  the  number  of 
spores  produced  each  year  by  a  well  grown  specimen  of  the 
shield  fern  (Nephrodium  filix-mas)  at  from  50,000,000  to 
100,000,000,  while  the  estimate  for  the  fern  Angiopteris  has 
been  placed  at  4,000,000,000  spores  for  a  single  leaf.  One 
plant  may  have  as  many  as  50  or  more  spore-bearing 
leaves.  It  has  been  pointed  out  that,  at  these  rates  of 
increase,  unrestricted,  a  given  species  of  plant  would,  in 
two  or  three  years,  cover  an  area  several  thousand  times 
that  of  the  dry  land.  But  nothing  of  the  sort  occurs. 
There  must,  therefore,  be  an  intense  struggle  for  existence, 
in  which  the  vast  majority  of  individuals  perish.  Darwin1 
gives  the  following  illustration : 

"  Seedlings,  also,  are  destroyed  in  vast  numbers  by 
various  enemies;  for  instance,  on  a  piece  of  ground  3 
feet  long  and  2  wide,  dug  and  cleared,  and  where  there 
could  be  no  choking  from  other  plants,  I  marked  all  the 
seedlings  of  our  native  weeds  as  they  came  up,  and  out  of 
357  no  less  than  295  were  destroyed,  chiefly  by  slugs  and 

1  "Origin  of  Species"  (New  York,  1902  edition),  pp.  83,  84. 


96  HEREDITY   AND    EVOLUTION   IN   PLANTS 

insects.  If  turf  which  has  long  been  mown,  and  the  case 
would  be  the  same  with  turf  closely  browsed  by  quadru- 
peds, be  let  to  grow,  the  more  vigorous  plants  gradually 
kill  the  less  vigorous,  though  fully  grown  plants;  thus  out 
of  20  species  growing  on  a  little  plot  of  mown  turf  (3  feet 
by  4)  nine  species  perished,  from  the  other  species  being 
allowed  to  grow  up  freely." 

" Struggle  for  Existence'1  Used  in  a  Large  Sense. — "I 
should  premise,"  said  Darwin,  "that  I  use  this  term  in  a 
large  and  metaphorical  sense  including  dependence  of  one- 
being  on  another,  and  including  (which  is  more  important) 
not  only  the  life  of  the  individual,  but  success  in  leaving 
progeny.  Two  canine  animals,  in  a  time  of  dearth,  may 
be  truly  said  to  struggle  with  each  other  which  shall  get 
food  and  live.  But  a  plant  on  the  edge  of  a  desert  is  said 
to  struggle  for  life  against  the  drought,  though  more 
properly  it  should  be  said  to  be  dependent  on  the  moisture. 
A  plant  which  annually  produces  a  thousand  seeds,  of 
which  only  one  on  an  average  comes  to  maturity,  may  be 
more  truly  said  to  struggle  with  the  plants  of  the  same  and 
other  kinds  which  already  clothe  the  ground.  The  mistle- 
toe is  dependent  on  the  apple  and  a  few  other  trees,1  but 
can  only  in  a  far-fetched  sense  be  said  to  struggle  with 
these  trees,  for,  if  too  many  of  these  parasites  grow  on  the 
same  tree,  it  languishes  and  dies.  But  several  seedling 
mistletoes,  growing  close  together  on  the  same  branch,  may 
more  truly  be  said  to  struggle  with  each  other.  As  the 
mistletoe  is  disseminated  by  birds,  its  existence  depends 
on  them;  and  it  may  metamorphically  be  said  to  struggle 
1  In  the  above  quotation,  Darwin  is  undoubtedly  referring  to  the 
European  mistletoe  (Viscum  album).  The  American  mistletoe  (Phora- 
dendron  flavescens)  is  found  in  the  eastern  and  central  United  States  on 
various  deciduous-leaved  trees,  including  the  sour  gum  and  red  maple. 


DARWINISM  97 

with  other  fruit-bearing  plants,  in  tempting  the  birds  to 
devour  and  thus  disseminate  its  seeds.  In  these  several 
senses,  which  pass  into  each  other,  I  use  for  convenience 
sake  the  general  term  of  Struggle  for  Existence." 

5.  Survival  of  the  Fittest. — In  this  struggle  for  existence 
only  those  best  suited  to  their  environment  will  survive. 
The  dandelion  from  the  seed  that  germinates  first  secures 
the  best  light;  the  one  that  sends  down  the  longest  and 
most  vigorous  root-system,  that  produces  the  largest,  most 
rapidly  growing  leaves  will  survive,  and  will  tend  to  trans- 
mit its  vigorous  qualities  to  its  progeny.  Less  vigorous 
or  less  "fit"  individuals  perish.  To  this  phenomenon 
Herbert  Spencer  applied  the  phrase,  "survival  of  the  fit- 
test." Darwin  called  it  "natural  selection,"  because  it 
was  analogous  to  the  artificial  selection  of  favored  types 
by  breeders  of  plants  and  animals.  It  will  be  readily  seen, 
however,  that  the  process  in  nature  is  not  so  much  a  selec- 
tion of  the  fittest,  as  a  rejection  of  the  unfit;  the  unfit  are 
eliminated,  while  the  fit  survive.  It  has  been  suggested 
that  "natural  rejection"  would  be  a  better  name  than 
"natural  selection."  "Variations  neither  useful  nor  in- 
jurious," said  Darwin,  "would  not  be  affected  by  natural 
selection." 

78.  Difficulties  and  Objections.— The  publication  of 
Darwin's  "Origin  of  Species"  aroused  at  once  a  storm  of 
opposition.  Theologians  opposed  the  theory  because  they 
thought  it  eliminated  God.  Especially  bitter  antagonism 
was  aroused  by  Darwin's  suggestion  that,  by  means  of 
his  theory  "much  light  will  be  thrown  on  the  origin  of 
man  and  his  history."  The  unthinking  and  the  careless 
thinkers  accused  Darwin  of  teaching  that  man  is  descended 
from  monkeys.  Neither  of  these  accusations,  however, 


98  HEREDITY   AND   EVOLUTION   IN   PLANTS 

is  true.  Darwinism  neither  eliminates  God,  nor  does  it 
teach  that  monkeys  were  the  ancestors  of  men. 

By  slow  degrees,  however,  men  began  to  give  more  care- 
ful and  unprejudiced  attention  to  the  new  theory,  and  not 
to  pass  adverse  judgment  upon  it  until  they  were  sure  they 
understood  it.  "A  celebrated  author  and  divine  has 
written  to  me,"  says  Darwin,  "that  he  has  gradually 
learnt  to  see  that  it  is  just  as  noble  a  conception  of  the 
Deity  to  believe  that  He  created  a  few  original  forms  capa- 
ble of  self-development  into  other  and  needful  forms,  as 
to  believe  that  He  required  a  fresh  act  of  creation  to  supply 
the  voids  caused  by  the  action  of  His  laws." 

And  in  closing  his  epoch-making  book,  Darwin  called 
attention  to  the  fact  that,  in  the  light  of  evolution,  all 
phases  of  natural  science  possess  more  interest  and  more 
grandeur. 

"When  we  no  longer  look  at  an  organic  being  as  a  savage 
looks  at  a  ship,  as  something  wholly  beyond  his  compre- 
hension; when  we  regard  every  production  of  nature  as 
one  which  has  had  a  long  history;  when  we  contemplate 
every  complex  structure  and  instinct  as  the  summing  up 
of  many  contrivances,  each  useful  to  the  possessor,  in  the 
same  way  as  any  great  mechanical  invention  is  the  sum- 
ming up  of  the  labour,  the  experience,  the  reason,  and  even 
the  blunders  of  numerous  workmen;  when  we  thus  view 
each  organic  being,  how  far  more  interesting — I  speak  from 
experience — does  the  study  of  natural  history  become!" 

"It  is  interesting  to  contemplate  a  tangled  bank,  clothed 
with  many  plants  of  many  kinds,  with  birds  singing  on  the 
bushes,  with  various  insects  flitting  about,  and  with  worms 
crawling  through  the  damp  earth,  and  to  reflect  that  these 
elaborately  constructed  forms,  so  different  from  each 


DARWINISM  99 

other,  and  dependent  upon  each  other  in  so  complex  a 
manner,  have  all  been  produced  by  laws  acting  around  us. 
These  laws,  taken  in  the  largest  sense,  being  Growth  with 
Reproduction;  Inheritance  which  is  almost  implied  by 
reproduction;  Variability  from  the  indirect  and  direct 
action  of  the  conditions  of  life,  and  from  use  and  disuse; 
a  Ratio  of  Increase  so  high  as  to  lead  to  a  Struggle  for  Life, 
and  as  a  consequence  to  Natural  Selection,  entailing  Diver- 
gence of  Character  and  the  Extinction  of  less-improved 
forms.  Thus,  from  the  war  of  nature,  from  famine  and 
death,  the  most  exalted  object  which  we  are  capable  of 
conceiving,  namely,  the  production  of  the  higher  animals, 
directly  follows.  There  is  grandeur  in  this  view  of  life, 
with  its  several  powers  having  been  orginally  breathed 
by  the  Creator  into  a  few  forms  or  into  one;  and  that, 
whilst  this  planet  has  gone  cycling  on  according  to  the 
fixed  law  of  gravity,  from  so  simple  a  beginning  endless 
forms  most  beautiful  and  most  wonderful  have  been,  and 
are  being  evolved." 

79.  Objections  from  Scientists. — Objections  to  Dar- 
win's .theory  were  also  brought  forward  by  scientific  men — 
partly  from  prejudice,  but  chiefly  because  they  demanded 
(and  rightly)  more  evidence,  especially  on  certain  points 
which  seemed  at  variance  with  the  theory.  For  example, 
they  said,  no  one  has  ever  observed  a  new  species  develop 
from  another;  this  ought  to  be  possible  if  evolution  by 
natural  selection  is  now  in  progress.  The  absence  of  - 
"connecting  links,"  or  transitional  forms  between  two 
related  species  was  noted;  the  presence  of  apparently 
useless  characters  (of  which  there  are  plenty  in  both 
animals  and  plants)  was  not  accounted  for;  and  the 
geologists  and  astronomers  claimed  that  the  time  required 


100  HEREDITY   AND   EVOLUTION   IN   PLANTS 

for  evolution  to  produce  the  organic  world  as  we  now  behold 
it  is  longer  than  the  age  of  the  earth  as  understood  from 
geological  and  astronomical  evidence. 

There  is  not  space  here  to  summarize  the  answers  to  all 
these  objections.  Suffice  it  to  say  that  scientific  investi- 
gation since  Darwin's  time  has  given  us  reasonably  satis- 
factory answers  to  most  of  them,  so  that  now  practically 
no  scientific  man  doubts  the  essential  truth  of  evolution; 
it  is  the  corner  stone  of  all  recent  science,  the  foundation 
of  all  modern  thought. 

80.  The  Modern  Problem.— But  Darwinism  left  us 
with  a  very  large  and  very  fundamental  problem  unsolved. 
Upon  what  materials  does  natural  selection  act  in  the 
formation  of  species?  Obviously  the  "fittest"  survives, 
but  what  is  the  origin  of  the  fittest?  This  problem  Darwin- 
ism did  not  solve.  The  solution  of  it  is  one  of  the  most 
fundamental  and  important  tasks  now  being  undertaken 
by  biologists.  The  most  effective  attack  is  by  the  method 
of  experimental  evolution,  which  forms  the  subject  of  the 
next  chapter. 


CHAPTER  VIII 
EXPERIMENTAL  EVOLUTION 

81.  A  New  Method  of  Study. — Previous  to  Darwin's 
time  the  study  of  plants  and  animals,  was  carried  on  chiefly 
by  observations  in  the  field.     The  science  was  largely 
descriptive — a  record  of  what  men  had  observed  under 
conditions  over  which  they  did  not  endeavor  to  exercise 
any  control;  it  was  accurately  named  " Natural  History" 
— a  description  of  Nature.     But  Darwin  and  a  few  of  his 
contemporaries,    especially   among   botanists,    began   to 
make  observations  under  conditions  which  they  determined 
and  largely  regulated.     In  this  way  the  problems  were 
simplified,  observation  became  more  accurate,  and  the 
endeavor  was  made  to  assign  the  probable  causes  of  the 
observed    phenomena.     With    the    introduction  of  this 
experimental  method,  science  began  to  make  rapid  strides, 
and,  more  than  ever  before,  facts  began  to  be,  not  only 
recorded,  but  interpreted  and  explained. 

82.  Hugo  de  Vries. — The  director  of  the  Botanic  Gar- 
den in  Amsterdam,  Holland,  Hugo  de  Vries,  was  among 
the  first  to  demonstrate  that  the  method  of  experiment 
may  be  applied  to  the  study  of  the  origin  of  species.     His 
plan  was  to  secure  seed  of  a  given  species  from  a  plant 
which  he  believed  to  be  pure  with  reference  to  a  given 
character,  that  is,  not  contaminated  or  mixed  by  being 
cross-pollinated   with   another  variety   or  species.     The 


102  HEREDITY   AND    EVOLUTION   IN   PLANTS 

characters  of  the  parent  plant  were  carefully  noted  and 
recorded  by  photographs  and  written  descriptions,  and  by 
preserving  dried  and  pressed  herbarium  specimens.  The 
plants  of  the  second  generation  were  carefully  guarded 
from  being  cross-pollinated,  and  thus  "pure"  seed  were 
secured  for  a  third  generation.  This  was  continued  often 
for  25  or  30  generations  of  the  plant,  requiring  as  many 


FIG.  49. — Hugo  de  Vries.  His  pioneer  studies  of  osmosis  resulted  in 
fundamental  contributions  to  our  krfowledge  of  that  subject;  his  mutation- 
theory  is  one  of  the  most  important  contributions  to  the  study  of  evolu- 
tion since  Darwin. 

years  when  a  species  produced  only  one  crop  of  seed  a 
year.  Very  careful  records  and  preserved  specimens  were 
kept  of  the  plants  of  each  generation,  and  accurate  com- 
parisons were  made  to  see  if  any  individuals  showed  a 
tendency  to  vary  widely  from  their  parents  in  any  sig- 
nificant way,  such  as  showing  entirely  new  characters,  not 
expressed  in  the  parents,  or  failing  to  manifest  one  or  more 
of  the  characters  of  the  parent. 


EXPERIMENTAL  EVOLUTION 


103 


83.  Two  Kinds  of  Variation.— One  of  the  first  results 
of  de  Vries's  painstaking  work  was  the  demonstration  of 
what  he  believed  to  be  a  fundamental  difference  between 
two  distinct  kinds  of  variation — continuous  (or  fluctuating) 
and  discontinuous  (or  saltative,  i.e.,  leaping). 

84.  Continuous    Variation. — Continuous    variation    is 
quantitative — a  case  merely  of  more  or  less.     It  deals  with 
averages.     Some  flowers  on  a  red-flowered  plant  may  be 
lighter  or  darker  red,  but,  in  a  series  of  generations,  the 


of 


FIG.  50. — Fluctuating  variation  in  the  leaves  of  an  oak  (Quereiis  chry- 
solepis),  a,  all  the  leaves  of  a  twig;  b,  younger  leaves  of  a  twig;  c,  con- 
secutive leaves;  d,  some  leaves  on  one  season's  growth  of  a  twig.  (After 
Copeland.) 

average  of  a  large  number  in  each  generation  does  not 
vary,  and  the  departure  from  the  average  never  exceeds 
certain  limits.  The  flowers  of  a  given  species  may  have  a, 
certain  characteristic  odor,  but  the  odor  may  be  stronger 
in  some  flowers  than  in  others,  or  in  some  individual 
plants  than  in  others .  The  plants  grown  from  a  handful  of 
beans  of  the  same  variety  may  vary  in  height  within 
limits,  but  the  average,  height  of  a  large  number  will  not 
vary  in  successive  generations,  and  will  be  characteristic 


104  HEREDITY   AND    EVOLUTION   IN   PLANTS 

of  the  species  or  variety.  In  other  words,  continuous  or 
fluctuating  variation  is  variation  about  a  mean.  It  may 
be  illustrated  by  the  bob  of  a  swinging  pendulum,  which 
continually  fluctuates  within  definite  limits  about  the 
mean  position  assumed  when  the  pendulum  is  at  rest 
(Fig.  56). 

All  plants  and  animals  manifest  fluctuating  variation 
in  all  their  characters  (Fig.  50),  and  such  variations  are 
largely,  if  not  entirely,  dependent  upon  the  environment. 
A  slight  change  in  the  kind  of  food  elements  supplied,  or 
in  the  amount  of  water  or  sunlight  available  will  make  the 
leaves  or  petals  a  deeper  or  a  paler  color.  Rich  soil,  fa- 
voring a  more  abundant  food  supply,  will  cause  a  greater 
average  growth  than  poor  soil,  but  unless  the  seed  for 
future  generations  is  selected  from  the  tallest  plants, 
and  the  richness  of  the  soil  is  maintained,  the  plants  will 
revert  to  their  normal,  lower  average  of  height.  In  other 
words,  the  average  height  of  the  plants  of  any  given  variety 
is  a  constant  (unvarying)  character,  except  that  it  may  be 
temporarily  altered  by  careful  selection  of  seeds  from  the 
tallest  or  shortest  individuals,  or  by  choosing  the  largest 
or  the  smallest  seeds  from  any  given  plant,  or  by  making 
the  soil  richer  or  poorer,  or  otherwise.  When  the  selection 
ceases,  and  the  soil  is  maintained  at  average  fertility,  the 
characteristic  average  height  of  the  plants  is  restored. 

85.  Illustrations  of  Continuous  Variation. — In  a 
quart  of  beans,  for  example,  there  are  no  two  seeds  of 
precisely  the  same  proportion  or  size;  some  are  longer, 
some  shorter.  De  Vries  describes1  an  experiment  in 
which  about  450  beans  were  chosen  from  a  quantity 
purchased  in  the  market,  and  the  lengths  of  the  indi- 

xDe  Vries.     "The  Mutation  Theory,"  vol.  2,  p.  47,  Chicago,  1909, 


EXPERIMENTAL   EVOLUTION 


105 


viduals    measured.     The    length    varied   from    8  to    16 
millimeters,  and  in  the  following  proportions : 

ii   12   13  14  15  16 

108  167  106  33   7  i 


Millimeters  .......    8     9     10 

Number  of  beans,    i     2     23 


The  beans  were  then  placed 
in  a  glass  jar  divided  into 
nine  compartments,  all  the 
beans  of  the  same  length 
in  the  same  compartments. 
When  this  was  done  it  was 
found  that  the  beans  were 
so  grouped  that  the  tops  of 
the  columns  in  the  various 
compartments  followed  a 
curve,  known  as  Quetelet's1 
curve  (Fig.  51). 

This  curve  may  be  plotted 
by  erecting  vertical  lines 
(ordinates)  at  intervals  of  one 
millimeter  on  a  horizontal 
line  or  base,  the  height  of 
each  vertical  line  being  pro- 
portionate to  the  number  of 
beans  having  the  length  in-  FlG  SI._Demonstration  of 

dicated  in  figures  at  its  base.    Quetelet's  law  of  fluctuating  varia- 

This  curve  shows  the  freq-  bilit>'  in  the  lensth  of  seeds  of  * 


uency  of  occurrence  of  seeds 


common   bean   (Phaseolus   vulgaris). 


of 


any    given 


Description  in  the  text.     (Redrawn 
dimension   from  de  Vries.) 


1  So  named  from  its  discoverer,  Quetelet  (Ket-lay).  As  de  Vries  states: 
"For  a  more  exact  demonstration  a  correction  would  be  necessary,  since 
obviously  the  larger  beans  fill  up  their  compartment  more  than  a  similar 
number  of  small  ones." 


106  HEREDITY   AND    EVOLUTION   IN   PLANTS 

between  the  two  limits  or  extremes,  and  is  therefore  often 
referred  to  as  a  curve  of  frequency.  It  should  be  noted 
that,  in  the  case  illustrated,  the  greatest  frequency  (in- 
dicated by  the  highest  point  of  the  curve)  very  nearly 
coincides  with  the  average  dimension;  in  other  words,  the 
more  any  given  character  departs  from  the  average  for  that 
character,  the  less  frequent  is  its  occurrence. 

In  another  experiment,  ears  of  corn,  harvested  from 
the  same  crop,  were  measured  and  found  to  vary  in  length 


FIG.  52. — Curve  of  fluctuating  variation  (Quetelet's  curve),  formed  by 
arranging  82  ears  of  corn  in  ten  piles,  according  to  the  length  of  the  ears. 
The  extremes  were  4.5  and  9  inches.  The  ears  were  taken  from  unselected 
material  from  a  field  of  corn.  (After  Blakeslee.) 

from  4^  inches  to  9  inches;  the  largest  number  of  ears 
(20)  were  7  inches  long.  The  greater  the  departure  from 
this  length,  in  either  direction,  the  fewer  the  individuals; 
for  the  lengths  4  inches  and  9  inches  the  frequency  was 
zero.  When  the  ears  were  arranged  in  piles  according 
to  their  length,  the  tops  of  the  piles  indicated  the  curve 
of  frequency  (Fig.  52). 

The  curve  of  frequency  indicates  the  quantitative  dis- 
tribution of  any  character  or  quality  when  its  occurrence 
is  dependent  largely  upon  chance.  This  is  strikingly 


EXPERIMENTAL   EVOLUTION 


FIG.  53. — Photograph  of  beans  rolling  down  an  inclined  plane  and 
accumulating  at  the  base  in  compartments,  which  are  closed  in  front  by 
glass.  The  exposure  was  long  enough  to  cause  the  moving  beans  to  appear 
as  caterpillar-like  objects  hopping  along  the  board.  If  we  assume  that 
the  irregularity  of  shape  of  the  beans  is  such  that  each  may  make  jumps 
either  toward  the  right  or  toward  the  left  in  rolling  down  the  board,  the 
laws  of  chance  lead  us  to  expect  that  in  very  few  cases  will  these  jumps 
be  all  in  the  same  direction,  as  indicated  by  the  few  beans  collected  in  the 
compartments  at  the  extreme  right  and  left.  Rather  the  beans  will  tend 
to  jump  in  both  right  and  left  directions,  the  most  probable  condition 
being  that  in  which  the  beans  make  an  equal  number  of  jumps  to  the  right 
and  to  the  left,  as  shown  by  the  large  number  accumulated  in  the  central 
compartment.  If  the  board  be  tilted  to  one  side,  the  curve  of  beans  would 
be  altered  by  this  one-sided  influence.  In  like  fashion,  a  series  of  factors — 
either  of  environment  or  of  heredity — it'  acting  equally  in  both  favorable 
and  unfavorable  directions,  will  cause  a  collection  of  ears  of  corn  to  assume 
a  similar  variability  curve,  when  classified  according  to  their  relative  size. 
Such  curves,  called  QuStelet's  curves,  are  used  by  biometricians  in  classi- 
fying and  studying  variations  in  plants  and  animals.  (Photo  by  A.  F. 
Blakeslee.  Legend  slightly  modified  from  Journal  of  Heredity,  June, 
1916.) 


108  HEREDITY   AND    EVOLUTION   IN  PLANTS 

illustrated  by  the  grouping  of  bean  seeds  rolled  down  a 
smooth  inclined  plane,  and  collected  in  receptacles  at  the 
bottom  (Fig.  53).  The  seeds  are  started  rolling  midway 
between  the  edges  of  the  plane;  the  chances  are  about 
equal  for  some  of  the  seeds  to  fall  into  the  outside  compart- 
ments, but  the  odds  are  vastly  in  favor  of  their  landing  at 
or  near  the  center.  Thus  they  group  themselves  so  that 
the  tops  of  the  piles  form  a  curve  of  chance  variation. 
When  the  result  is  influenced  in  one  direction  more  than 
in  another  the  crest  of  the  curve  will  be  nearer  one  extreme 
than  the  other,  and  the  curve  is  to  that  extent  skew.  The 
curve  of  bean  seeds  in  Fig.  53  is  slightly  skew  toward 
the  right-hand  extreme.  Suggest  one  or  more  reasons 
why. 

86.  Fluctuating  Variation  and  Inheritance.— When 
the  ancestry  is  not  mixed  or  hybrid  the  curve  of  frequency 
of  any  character  in  one  generation  ordinarily  tends  to 
recur  in  successive  generations  of  descendants,  providing 
the  environment  remains  essentially  the  same. l 

87.  Discontinuous  Variation.— Long  before  Darwin, 
students  of  plants  and  animals  had  observed  a  different 
kind  of  variation  than  continuous— one  which  was  not 
quantitative  but  qualitative,  resulting  in  the  expression  of 
new  characters,  or  of  a  new  curve  of  frequency]  that  is,  in 
fluctuation  about  a  new  mean.     Plants  from  some  of  the 
seeds  of  a  red-flowered  specimen  bear  flowers,  not  that 
vary  from  deeper  to  paler  red,  but  that  suddenly,  at  one 
step,  have  become  pure  white;  one  or  more  seeds  from 
an  odorless  plant   may  give  rise  to  individuals  whose 
flowers  are  sweet-scented;  or  vice  versa,  odorless  specimens 

1  The  behavior  of  hybrid  descendants  is  a  special  case  described  in 
Chapter  XXXVII. 


EXPERIMENTAL   EVOLUTION 


ICQ 


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1 

1 

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1 

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/ 

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I 

k 

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55 

o'% 

^K 

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40-45                          TO-75 
-40                                           75 

80-85 
80     85 

90-95  1  10 
•90     95-100 

FIG.  54. — Curves  of  variation,  illustrating  the  difference  between 
fluctuation  about  a  given  mean,  and  the  appearance  of  a  new  mean,  i.e., 
mutation.  At  the  right,  variations  in  the  stature  of  Oenothera  Lamarckiana; 
at  the  left,  variation  in  the  stature  of  Oenothera  nanella,  a_form  derived 
from  O.  Lamarckiana  by  mutation.  (After  de  Vries.) 


FIG.  55. — Leaves  of  varieties  of  the  Boston  fern  (Nephrolejris),  showing 
(from  left  to  right)  progressive  branching  of  the  pinnae  and  pinnules,  and 
illustrating  so-called  "  orthogenetic  saltation."  (After  R.  C.  Benedict.) 


110  HEREDITY   AND   EVOLUTION   IN   PLANTS 

may  spring  at  one  leap,  not  by  gradual  changes,  from  those 
that  are  fragrant;  in  one  generation  the  factors  controlling 
height  are  so  altered  that,  in  successive  generations,  the 
average  of  height  may  change  by  either  more  or  less,  so 
that  the  heights  of  the  individuals  fluctuate  about  a  new 
mean.  In  other  words,  we  recognize  a  second  type  of 
variation — not  the  fluctuation  of  individuals  about  an 
unchanging  mean,  but  the  appearance  of  a  new  mean, 
about  which  the  given  character  in  individuals  may 
fluctuate  (Fig.  54.) 

When  discontinuous  variation  proceeds  along  a  definite 
line  through  several  successive  generations,  each  step  being 
an  intensification  of  the  preceding  one,  it  is  designated 
"  or  tho  genetic  saltation"  (Fig.  55). 

88.  Illustration  of  the  Pendulum. — The  difference  be- 
tween discontinuous  and  fluctuating  variation  may  be 
aptly  illustrated  by  a  swinging  pendulum  (Fig.  56).     The 
vertical  position,  assumed  when  at  rest,  is  i  he  mean  of  all 
positions  that  may  be  assumed  as  the  pendulum  swings; 
the  oscillation  about  this  mean  illustrates  continuous  or 
fluctuating  variation. 

But  we  may  conceive  that  the  point  of  suspension  of  the 
pendulum  changes,  as  shown  in  the  figure.  The  pendulum 
continues  to  oscillate,  but  now  about  a  new  mean  position ; 
a  new  character  has  been  introduced,  with  its  own  fluctua- 
tions of  more  or  less. 

89.  Mutations. — Darwin,  as  well  as  others  before  and 
after  him,  recognized  both  kinds  of  variation,  but  de  Vries 
was  the  first  to  work  out  in  detail  the  hypothesis  that 
discontinuous  variations  furnish  the  material  for  natural 
selection.     Discontinuous  variations  he  called  mutations; 
plants  which  give  rise  to  or  "throw"  them  are  said  to 


EXPERIMENTAL    EVOLUTION 


mutate.  A  plant  that  arises  by  mutation  is  an  elementary 
species,  or  mutant;  and  the  theory  that  mutations  (and  not 
fluctuations)  explain  the  origin  of  the  fittest,  and  supply 
the  materials  upon  which  natural  selection  operates  in  the 


tre/ne 


Mean 

FIG.  56.— Diagram  to  illustrate  the  difference  between  fluctuating 
variation  and  mutation;  O,  original  point  of  suspension;  M,  new  point  of 
suspension  after  the  mutation  has  occurred. 

formation  of  new  species,   de  Vries  called  the  mutation 
theory. 

90.  Examples  of  Mutation.— The  kohlrabi,  cauliflower, 
and  other  horticultural  varieties  of  the  wild  cliff-cabbage 
(Fig.  57),  are  believed  to  be  mutants,  and  to  have  arisen, 


HEREDITY    AND   EVOLUTION   IN  PLANTS 


EXPERIMENTAL   EVOLUTION  113 

not  by  the  prolonged  selection  of  fluctuating  variations, 
but  at  one  step — in  one  generation — as  "sports"  of  the 
wild  Brassica  oleracea.  Strawberry  plants  without  run- 
ners, green  dahlias  and  green  roses,  the  common  seedless 
bananas  of  the  markets,  the  Shirley  poppies,  pitcher- 
leaved  ash  trees,  Pierson's  variety  of  the  Boston  fern, 


FIG.  58. — Clover  leaves  with  three  to  nine  leaflets,  illustrating  a  tend- 
ency to  mutate.  The  normal  clover  leaf  is  a  pinnately  compound  leaf 
with  three  leaflets.  Plants  with  leaves  having  five  to  nine  leaflets  con- 
stitute a  "half-race,"  i.e.  the  normal  character  is  active,  the  anomaly 
semi-latent.  (Photo  by  the  author;  specimens  from  cultures  of  G.  H. 
Shull.) 

5-9-  "leaved"  clovers  (Fig.  58),  white  black-birds  (and 
other  albinos,  including  albino  men),  moss-roses,  thornless 
cacti  and  thornless  honey-locusts,  red  sunflowers,  com- 
posites with  tubular  corollas  in  the  ray-flowers  (Fig.  48), 
and  the  innumerable  white  flowered  varieties  of  colored 
flowered  species,  are  all  illustrations  of  mutation.  Fre- 
quently the  mutative  change  occurs  in  a  lateral  bud,  pro- 


114  HEREDITY  AND   EVOLUTION   IN  PLANTS 

ducing  a  "bud-sport"  (Fig.  60).     Such  was  the  origin  of 
the  seedless  naval  orange  from  the  seed-bearing  orange. 

91.  The  Evening-primrose. — In  1886  de  Vries  began  to 
search  for  a  species  that  was  in  a  mutating  condition,  be- 


FIG.  59. — Yellow  daisy,  or  cone-flower  (Rudbeckia  s£.),  showing  varia- 
tions of  the  character  of  mutations  in  the  ray-  and  disc  flowers.  At  d 
the  normally  ligulate  corollas  are  tubular;  at  /  they  have  all  aborted, 
except  two;  at  h  many  of  the  normally  tubular  disc-flowers  have  become 
ligulate,  making  a  nearly  "double  flower."  (Photo  by  E.  M.  Kittredge.) 

lieving  that  any  given  species  is  at  some  periods  in  its 
history  more  labile  or  changeable  than  at  other  periods. 
After  a  long  search  he  found,  in  an  abandoned  potato  field 
at  Hilversum,  near  Amsterdam,  a  large  number  of  plants 
of  Lamarck's  evening-primrose  ((Enothera  Lamarckiana) 
(Fig.  61). 


EXPERIMENTAL   EVOLUTION  IIS 

"That  I  really  had  hit  upon  a  plant  in  a  mutable  period 
became  evident  from  the  discovery,  which  I  made  a  year 
later,  of  two  perfectly  definite  forms  which  were  immedi- 
ately recognizable  as  two  new  elementary  species.  One  of 
them  was  a  short-styled  form:  O.  brevistylis,  which  at  first 
seemed  to  be  exclusively  male,  but  later  proved  to  have 


FIG.  60. — A  plant  of  the  evening-primrose  (CEnothera  fnennis)  which, 
by  "bud  sporting,"  has  given  rise  (at  the  left)  to  a  branch  having  the 
characters  of  another  species. 

the  power,  at  least  in  the  case  of  several  individuals,  of 
developing  small  capsules  with  a  few  fertile  seeds.  The 
other  was  a  smooth-leaved  form  with  much  prettier  foliage 
than  O.  Lamarckiana,  and  remarkable  for  the  fact  that 
some  of  its  petals  are  smaller  than  those  of  the  parent  type, 
and  lack  the  emarginate  form  which  gives  the  petals  of 


Il6  HEREDITY  AND   EVOLUTION  IN   PLANTS 

Lamarckiana  their  cordate  character.     I  call  this  form 
O.  losvifolia." 

"When  I  first  discovered  them  (1887)  they  were  repre- 
sented by  very  few  individuals.  Moreover  each  form 
occupied  a  particular  spot  on  the  field.  O.  brevistylis 
occurred  quite  close  to  the  base  from  which  the  (Enothera 


FIG.  61. — Lamarck's  evening-primrose  ((Enothera Lamarckiana).    A  muta- 
ting species.     Cf.  Fig.  62.     (After  de  Vries.) 

had  spread;  O.  lavifolia  on  the  other  hand,  in  a  small 
group  of  10  to  12  plants,  some  of  which  were  flowering 
whilst  others  consisted  only  of  radical  leaves,  in  a  part  of 
the  field  which  had  not  up  to  that  time  been  occupied  by 
O.  Lamarckiana.  The  impression  produced  was  that  all 
these  plants  had  come  from  the  seeds  of  a  single  mutant. 


EXPERIMENTAL  EVOLUTION 


117 


Since  that  time,  both  the  new  forms  have  more  or  less 
spread  over  the  field"  (de  Vries). 

Another  mutant  of  0.  Lamarckiana  was  called  by 
de  Vries  (Enothera  gigas  (Fig.  62).  The  cells  of  this 
mutant  have  twice  as  many  chromosomes  as  the  parent 
form. 


FIG.  62. — Giant  evening-primrose  (Enothera  gigas,  a  mutant  from 
(Enotlteia  Lamarckiana,  originated  in  1895.  Cf.  Fig.  61.  (After  de 
Vries.) 

92.  The  Test  of  a  Mutation.— The  deciding  test  as  to 
whether  a  given  new  form,  arising  without  crossing  from 
a  form  that  has  bred  true  for  at  least  two  generations,  is 
really  a  mutant  or  merely  a  fluctuating  variant,  is  to  see 
if  it  breeds  true  to  seed  for  the  new  character  or  characters. 


Il8  HEREDITY  AND   EVOLUTION  IN  PLANTS 

If  it  does  it  is  a  mutant;  otherwise  it  is  not.  It  is  clear, 
therefore,  that  the  only  way  the  problem  can  be  followed 
out  is  by  experiment — hence  the  term  experimental  evolu- 
tion. The  next  step  for  de  Vries  to  take,  after  discovering 
the  two  forms  that  he  supposed  to  be  mutants,  was  to  breed 
them  in  carefully  guarded,  pedigreed  cultures  in  his 
garden,  and  also  to  breed  the  parent  form,  (Enothera  La- 
marckiana,  and  see  if  he  could  observe  the  two  forms  above 
mentioned,  or  other  mutants,  arise  from  seed  produced 
without  crossing  with  any  other  species. 

The  entire  story  of  this  classical  series  of  experiments 
is  too  long  to  be  told  here.  Suffice  it  to  say  that  de  Vries 
did  observe  numerous  other  aberrant  forms  arise,  and  also 
found  that  they  bred  true  (except  for  additional  muta- 
tions) when  propagated  by  seed  for  over  25  years — that 
is,  they  were  true  mutations. 

93.  Relation  of  Mutation  Theory  to  Darwinism. — The 
mutation  theory  is  not  intended  by  de  Vries  to  supplant 
the  theory  of  natural  selection,  but  to  demonstrate  that 
the  materials  upon  which  selection  acts  in  the  formation 
of  new  species  are  mutations,  and  mutations  only — never 
fluctuating  or  individual  variations.  Here  lies  the  essen- 
tial difference  between  Darwin  and  de  Vries,  for  Darwin, 
though  recognizing,  and  with  increasing  clearness,  that 
mutation  furnishes  part  of  the  material  to  be  "selected" 
by  nature,  assigned  a  larger  and  more  important  role  to 
fluctuating  or  individual  variations.  "Species  have  been 
modified,"  he  said,  "chiefly  through  the  natural  selection 
of  numerous  successive,  slight,  favorable  variations;  aided 
[however]  in  an  important  manner  by  ....  variations 
which  seem  to  us  in  our  ignorance  to  arise  spontaneously. 
It  appears  that  I  formerly  underrated  the  frequency  and 


EXPERIMENTAL   EVOLUTION  1 19 

value  of  these  latter  forms  of  variation,  as  leading  to 
permanent  modifications  of  structure  independently  of 
natural  selection.1  And  lie  goes  on  to  say  that,  "as  my 
conclusions  have  lately  been  much  misrepresented,  and  it 
has  been  stated  that  I  attribute  the  modification  of  species 
exclusively  to  natural  selection,  I  may  be  permitted  to 
remark  that  in  the  first  edition  of  this  work  (the  Origin), 
and  subsequently,  I  placed  in  a  most  conspicuous  position 
— namely,  at  the  close  of  the  Introduction — the  following 
words:  'I  am  convinced  that  natural  selection  has  been 
the  main  but  not  the  exclusive  means  of  modification.'"2 
In  the  second  place,  the  mutation  theory  explains  away 
numerous  objections  to  natural  selection.  It  shows  how 
characters  that  are  never  of  vital  importance3 — i.e.,  matters 
of  actual  life  or  death — to  a  species  may  arise  and  be  per- 
petuated. Without  mutation  this  is  difficult  to  explain,4 

1  Darwin,  C.     The  Origin  of  Species.     6  Ed.  New  York.     D.  Apple- 
ton  &  Co.,  1902,  p.  293. 

2  Darwin  almost  dispaired  of  making  his  position  on  this  point  under- 
stood.    The  clear  statement  above  quoted,  he  said,  "has  been  of  no 
avail.     Great  is  the  power  of  steady  misrepresentation;  but  the  history 
of  science  shows  that  fortunately  this  power  does  not  long  endure." 
Darwin.     1.  c.,  p.  293. 

3  As  required  by  Darwin's  theory.     See  quotation  above  (p.  118),  and 
on  p.  97. 

1  Other  explanations  have  been  offered.  For  example,  sometimes  two 
characters  appear  to  be  always  associated,  so  that  the  presence  of  one 
involves  the  presence  of  the  other;  as  a  mane  and  maleness  hi  the  lion, 
dicotyledony  and  exogeny  in  Angiosperms.  The  constant  association  of 
two  characters  is  often  (though  not  always)  due  to  the  fact  that  the  factors 
for  those  characters  tend  to  keep  together  in  the  same  chromosome,  in- 
stead of  segregating  during  the  formation  of  egg-cells  and  sperms.  This 
tendency  is  called  linkage.  The  association  of  smooth  (vs.  wrinkled)  seed 
with  tendrilled  (vs.  non-tendrilled)  leaves  in  the  garden  pea,  and  of  red 
flower-color  with  round  pollen  in  the  sweet  pea  may  be  cited  as  examples  of 
linkage.  In  such  cases  one  of  the  characters  might  be  of  vital  importance 
to  a  plant  in  the  struggle  for  existence  and  the  other  not. 


120  HEREDITY   AND   EVOLUTION  IN   PLANTS 

and  yet  many,  if  not  most,  of  the  characteristics  by  which 
different  species  are  distinguished  from  each  other  are  of 
this  kind — not,  so  far  as  we  can  see,  absolutely  essential 
to  the  life  of  the  species.  Mutation  also  offers  a  method 
by  which  evolutionary  changes  may  take  place  within  a 
much  shorter  time-period  than  was  demanded  by  the 
natural  selection  of  fluctuations.  Incidentally,  the  muta- 
tion theory  clearly  shows  that  the  absence  of  "  connecting 
links"  between  species  is  not,  as  was  formerly  urged,  an 
argument  against  evolution,  but  is,  on  the  contrary,  just 
what  we  might  expect  to  find. 

94.  Value  of  the  Mutation  Theory. — The  elaboration  of 
the  mutation  theory  (together  with  the  rediscovery  of 
Mendel's  law,  to  be  discussed  in  Chapter  V)  furnished  the 
biological  world  with  a  new  method  of  study;  it  demon- 
strated that  the  method  of  evolution,  so  far  as  it  concerns 
the  origin  of  new  characters,  may  be  studied  by  experi- 
mentation.1 The  mutation  theory  should  also  be  of  great 
service  to  breeders.  It  has  helped  to  establish  plant  and 
animal  breeding  on  a  more  scientific  basis,  has  pointed  the 
way  to  correct  methods  where  men  where  formerly  groping 
in  the  dark,  and  has  showed,  that  results  of  commercial 
value  do  not  require  a  life  time,  but  may  be  obtained  with- 
in two  or  three  seasons.  By  the  application  of  modern 
methods  it  has  been  possible,  within  a  few  seasons,  to 

1  Like  most  great  contributions  to  science,  the  elaboration  of  the  experi- 
mental method  of  approach  to  the  problem  of  heredity  and  evolution  cannot 
be  attributed  solely  to  any  one  man.  Students  of  science  in  any  period 
come  into  a  rich  inheritance  in  the  labors  of  many  predecessors.  To 
fully  assign  the  credit  for  the  experimental  method  in  the  study  of  heredity 
it  would  be  necessary  to  write  the  history  of  investigations  extending  from 
Kolreuter  (1760)  one  of  the  first,  if  not  the  first  hybridizer,  of  plants,  Knight 
(1799),  through  Gaertner  (1849),  Jordan  (1853),  Naudin  (1862),  and  others 
to  Mendel,  de  Vries,  and  those  of  more  recent  date,  down  to  our  own  time. 


EXPERIMENTAL   EVOLUTION  121 

obtain  new  strains  of  oats  yielding  as  much  as  14  bushels 
per  acre  more  than  the  variety  from  which  they  were  de- 
rived, and  to  produce  new  strains  of  corn  not  only  giving 
a  larger  yield,  but  maturing  nearly  two  weeks  earlier  than 
the  parent  variety. 


FIG.  63. — Linnaeus,  the  great  classifier  (1707-1778).  He  is  wearing  a 
sprig  of  the  twin-flower  (Linncea  borealis),  one  of  his  favorite  flowers,  and 
named  after  him  by  his  friend,  Gronovius.  He  is  regarded  as  the  father 
of  modern  systematic  botany. 

95.  Classification. — Mere  information  is  not  science. 
A  "book  of  facts"  is  not  a  scientific  treatise  for  it  is  com- 
posed of  bits  of  unrelated  information,  presented  on  some 
artificial  basis  of  sequence,  as  for  example,  alphabetically. 


122  HEREDITY  AND   EVOLUTION   IN  PLANTS 

Scientific  knowledge,  in  addition  to  being  as  accurate  as 
possible,  is  characterized  by  having  an  orderly  arrangement 
in  one's  mind,  and  this  order  is  based  on  a  logical,  funda- 
mental relationship  between  the  facts  and  ideas.  Only 
by  such  an  arrangement  of  our  ideas  are  we  able  to  under- 
stand their  relation  to  each  other,  their  relative  impor- 
tance, and  their  real  significance.  Classification,  there- 
fore, is  essential  to  all  science.  The  very  existence  and  use 
of  such  words  as  oaks,  maples,  roses,  indicate  that  men 
have  grouped  or  classified  their  ideas  of  certain  plants 
(e.g.,  red  oaks,  white  oaks,  black  oaks,  bur  oaks,  live 
oaks,  etc.),  and  have  thereby  recognized  that  certain  kinds 
resemble  each  other  closely  enough  to  be  placed  in  one 
group  with  a  group-name.  All  the  common  names  of 
plants  indicate  the  recognition  of  classes — a  classification. 
96.  Evolution  and  Classification. — Without  the  guiding 
idea  of  evolution  classification  would  be  arbitrary  and 
artificial.  Linnaeus  classified  plants  on  the  basis  of  the 
number  of  stamens  they  possessed,  thus  placing  in  one 
group  plants  now  known  to  be  wholly  unrelated,  except 
that  they  have  a  chance  similarity  in  the  number  of 
stamens.  In  like  manner  we  may  group  together  plants 
with  red  flowers,  blue  flowers,  or  pink  flowers,  as  is  often 
done  in  "popular"  guides  to  the  wild  flowers.  This  has 
its  value,  but  it  tells  us  really  nothing  about  the  significant 
relationship  between  plants,  does  not  help  clear  up  our 
own  ideas,  does  not  show  the  gaps  in  our  knowledge  and 
tell  us  where  to  search  for  new  facts  to  fill  up  the  gaps. 
Evolution,  by  showing  that  plants  are  all  related  to  each 
other  by  descent,  just  as  are  the  members  of  a  large  family 
of  persons,  discloses  to  us  the  only  true  basis  of  classifica- 
tion— the  plan  that  endeavors  to  arrange  all  plants  so  as  to 


EXPERIMENTAL   EVOLUTION  123 

show  their  descent  and  their  relationship  to  each  other. 
Without  evolution  there  might  be  any  number  of  arbitrary 
systems;  on  the  basis  of  evolution  there  can,  in  the  end, 
be  but  one  true  system,  which  all  students  must  accept, 
because  it  will  be  a  true  record  of  what  has  actually  oc- 
curred in  the  history  of  development  of  the  plant  or  animal 
world.  In  other  words,  if  our  knowledge  should  ever  be- 
come sufficiently  complete  and  exact,  the  classification  of 
plants  would  give  a  summary — a  bird's  eye  view — of  the 
course  of  evolution  and  the  history  of  development.  To 
approximate  this  end  is  one  of  the  largest  problems  of  botany. 


CHAPTER  IX 
THE  EVOLUTION  OF  PLANTS 

97.  The  Problem  Stated.— If  we  knew  the  entire 
history  of  development  of  the  plant  world,  we  could  ar- 
range all  plants  now  living,  and  that  have  lived,  so  as  to 
show  their  genetic  relation  to  each  other.  The  prob- 
lem is  illustrated  on  a  small  scale  by  various  related  culti- 
vated plants,  all  known  to  be  derived  from  a  common 
wild  ancestor.  Cabbage  and  its  relatives  are  a  case  in 
point.  The  botanical  relatives  of  the  cabbage  include 
such  forms  as  kohlrabi,  brussels-sprouts,  collards,  kale, 
broccoli,  and  cauliflower  (Fig.  44).  All  of  these  garden 
vegetables  are  believed  to  have  been  derived  from  the 
common  wild  cliff-cabbage  (Brassica  oleracea)  of  Europe 
and  Asia,  by  selecting  mutations  in  various  directions, 
e.g.,  excessive  development  of  the  stem  in  kohlrabi,  of 
the  terminal  bud  in  cabbages,  of  the  lateral  buds  in  brus- 
sel's  sprouts,  of  the  flower  buds  in  cauliflower.  Or,  to 
refer  to  de  Vries's  studies  in  experimental  evolution,  where 
the  course  of  descent  was  actually  observed,  we  may 
arrange  the  forms  of  Lamarck's  evening-primrose  so  as  to 
show  their  known  derivation. 

The  general  problem,  therefore,  is  to  establish  the 
genetic  relationship  of  all  known  plants,  living  and  fossil. 
Since  the  Angiosperms  stand  at  the  top  of  the  series,  the 
problem  resolves  itself  largely  into  ascertaining  the 
phytogeny,  or  line  of  ancestry,  of  that  group. 
124 


THE   EVOLUTION   OF   PLANTS  12$ 

98.  Methods  of  Study. — In  the  solution  of  this  prob- 
lem two  methods  of  attack  may  be  employed:  (i)  That 
of  observation  and  comparison  of  structure,  followed  by 
classification,  and  inference;  (2)  that  of  experiment.     The 
use  of  experiment  is  indicated  in  Chapters  V  and  VIII. 
By  this  means  we  may  learn  something  of  the  relationship 
of  different  groups  having  living  representatives,  but  it 
chiefly  serves  to  throw  light  on  the  method  of  evolution. 
The  course  of  evolution  is  best  ascertained  by  the  observa- 
tion and  comparison  of  plant  structures. 

99.  Sources  of  Evidence. — There  are  five  main  sources 
of  evidence  as  to  the  course  of  evolution: 

1.  Comparative  life  histories  of  living  forms. 

2.  Comparative  anatomy  of  living  forms. 

3.  Geographical  distribution. 

4.  Structure  of  fossil  forms. 

5.  Geological  succession  of  fossil  forms. 

Studies  along  these  five  different  lines  have  resulted 
in  some  conflict  of  evidence,  but  on  the  whole  the  evidence 
from  the  various  sources  all  points  to  the  same  broad 
conclusions.  Conflict  or  contradication  is  in  most  cases 
the  result  of  insufficient  evidence  from  one  or  more  sources. 

100.  Evidence  from  Life  Histories. — In  the  study  of 
the   life   history    (ontogeny]    of   any   higher   sporophyte, 
we  find  that  vegetative  (sterile)  tissues  develop  first.     On 
the  basis  of  this  fact  it  has  been  inferred  by  some  in- 
vestigators1 that  all  reproductive  organs  (stamens,  carpels, 
sporophylls)  arose  by  a  modification  of  vegetative  organs. 
Other  facts,  however,  as  set  forth  on  pages  126-129,  nave 
lead  to  the  directly  opposite  conclusion. 

1See  Bower,  F.  O.  "The  origin  of  a  land  flora."  Macmillan  and  Co. 
Ltd.,  London,  1908. 


126 


HEREDITY  AND   EVOLUTION   IN  PLANTS 


101.  Evidence  from  Comparative  Ontogeny. — In  zool- 
ogy, evidence  of  the  course  of  evolution  is  also  seen  in  the 
recapitulation  of  the  characters  of  lower  forms  in  the  em- 
bryogeny  of  higher  forms.  This  is  often  referred  to  as  von 
Baer's  law.  Evidence  of  that  nature  is  less  striking  and 
less  common  in  plants.  It  is  found,  however,  in  a  com- 
parison of  the  young  or  embryonic  stage  of  the  sporophyte 
of  the  higher  liverwort,  Marchantia,  with  the  mature 


FIG.  64. — The  apical  cell  in  the  stem  apex  in  various  phyla,  from 
Bryophytes  to  Gymnosperms.  A,  acrogynous  liverwort  (Notothylus 
orbkularis)]  B  &  C,  eusporangiate  ferns  (B,  Maraitia  Douglasii,  C, 
Ophioglossum  pendulum) ;  D  &E,  homosporous  leptosporangiate  ferns  (D, 
Osmunda  Claytoniana,  E,  Adiantum  emarginatum,  representing  Polypodi- 
ales);  F,  heterosporous  leptosporangiate  fern  (Marsilia  vestita)]  G,  a 
horsetail  (Calamophyte)  (Equisetum  telamateia) ;  H,  a  late  gymnosperm 
(Pinus  Laricio).  (A-G  redrawn  from  Campbell,  H  from  Buchholz). 

sporophyte  of  the  lower  liverwort  Riccia  (Fig.  65).  The 
latter  consists  almost  entirely  of  "fertile"  (i.e.,  reproduc- 
tive) cells.  As  we  pass  to  more  highly  organized  forms, 
such  as  Marchantia,  the  relative  amount  of  vegetative 
tissue  gradually  increases  by  a  progressive  sterilization1  of 
fertile  tissue.  This  progressive  sterilization  is  repeated 
in  the  ontogeny  of  the  sporophytes  of  the  higher  forms. 
The  thread-like,  green  protonema  of  mosses  is  often  in- 
1  See  foot-note,  p.  125. 


THE  EVOLUTION  OP  PLANTS  127 

terpreted  as  reminiscent  of  an  ancestral  filamentous  green 
alga,  and  the  appearance  in  the  embryo  of  pines  and 
other  conifers  of  a  larger  number  of  primordia  than  of 
mature  cotyledons,  has  also  been  regarded  as  a  re- 
capitulation of  an  ancestral  feature  (Fig.  104).  Bucholz1 
has  demonstrated  that  young  pine  embryos  possess 
an  apical  cell  similar  to  that  characteristic  of  ferns 
and  fern-allies,  this  apical  cell  persisting  until  the  pine 
embryo  comprises  several  hundred  cells,  and  then  loosing 
its  identity  (Fig.  64). 

102.  Evidence  from  Comparative  Anatomy. — Compara- 
tive study  of  structure  has  led  to  the  conclusion  that, 
in  its  broadest  aspects,  the  course  of  plant  evolution  has 
been  from  the  simple  to  the  complex;  that  such  simple 
organisms  as  Pleurococcus,  and  other  green  algae,  preceded 
more  complex  forms  like  the  liverworts;  that  Bryophytes 
probably  appeared  before  ferns,  and  they  in  turn  before 
the  modern  Gymnosperms  and  Angiosperms. 

A  difficulty  of  accepting  this  conclusion  as  final  is  the 
possibility  that,  at  certain  points,  the  course  of  evolution 
may  have  been  retrograde — i.e.,  from  the  more  complex 
to  the  less  complex.  For  example,  it  is  generally  accepted 
that  the  filamentous,  alga-like  fungi  were  derived  from 
green  algae  by  retrograde  evolution  (degeneration).  Were 
the  plants  with  one  seed-leaf  (monocotyledons)  derived 
from  those  with  two  (dicotyledons)  by  retrograde  evolu- 
tion, or  were  the  dicotyledons  derived  from  the  monocoty- 
ledons by  progressive  evolution?  Evidence  ascertained 
by  comparative  studies  of  vascular  anatomy  and  other  de- 
tails of  structure  points  to  the  conclusion,  that,  although 

1Bucholz,  J.  T.  Suspensor  and  Early  Embryo  of  Finns.  Bot.  Gaz. 
66:  185-228.  Sept.,  1918. 


128 


HEREDITY   AND    EVOLUTION   IN   PLANTS 


monocotyledony  seems  the  simpler,  more  primitive  condi- 
tion, it  is  really  a  later  phenomenon,  the  monocotyledons 
being  derived  from  the  dicotyledons  by  simplification.1 

As  a  further  example  there  may  be  cited  the  application 
of  the  method  of  comparative  anatomy  to  solve  the  problem 


FIG.  65. — Progressive  sterilization  of  tissue  in  spprophytes.  a,  Riccia 
trichocarpa  (mature) ;  b,  Marchantia  polymorpha  (embryo) ;  c,  Marchantia 
(mature);  d,  Porrella,  a  leafy  liverwort  (mature);  e,  anthoceros;  /,  Ly co- 
podium  Selago;  g,  Lycopodium  complanatum;  h,  Botrychium  Lunaria 
(Eusporangiate);  i,  Polypodium  venosum  (Leptosporangiate).  (Re- 
drawn from  various  sources.) 

of  the  origin  of  the  leafy  sporophyte.  As  noted  above 
(Ifioi),  the  most  primitive  spore-producing  phases  (sporo- 
phytes)  of  the  lower  liverworts  (Hepaticae)  consist 
almost  entirely  of  "fertile"  (i.e.,  reproductive)  cells, 
and  the  relative  amount  of  vegetative  or  sterile  tissue 
1  See  page  223. 


THE  EVOLUTION  OF  PLANTS  I2Q 

gradually  increases,  as  we  pass  to  more  highly  organized 
forms,  indicating  a  progressive  sterilization  of  the  fertile 
tissue  during  evolutionary  development.  A  survey  of 
the  sporophytic  phases  of  the  liverworts,  mosses,  and 
ferns  will  show  how  these  sporophytes  gradually  in- 
crease in  complexity  and  importance,  from  the  simple 
condition  in  the  liverwort  Riccia,  with  almost  no  sterile 
tissue,  through  the  sporogonium  of  the  higher  liverworts 
and  mosses,  to  the  leafy  sporophyte  of  the  ferns  (Fig.  65). 
The  final  step  in  the  development  of  the  sporophyte  was 
the  differentiation  of  plants  bearing  only  large  spores 
(megasporophytes),  and  those  bearing  only  small  spores 
(microsporophyies),  represented  in  the  Angiosperms  re- 
spectively by  the  pistillate  and  staminate  plants.  The 
progressive  sterilization  accompanied  a  change  in  the 
habitat  of  the  plants  from  water  to  dry  land.1 

On  the  other  hand,  a  careful  student  of  fossil  plants 
has  recently  been  led  to  state  that,  "it  is  beginning  to 
appear  more  probable  that  the  Higher  Cryptogams  (ferns 
and  fern  allies)  are  a  more  ancient  and  primitive  group 
than  the  Bryophytes,  which  would  seem  to  owe  their 
origin  to  reduction  from  some  higher  type."2  In  view  of 
this  diversity  of  opinion,  we  learn  at  once  that  great  cau- 
tion must  be  used  in  interpreting  the  evidence — that  we 
must  not  "jump  at  conclusions." 

103.  Results  of  the  Method  of  Comparative  Anatomy. 
By  their  study  of  comparative  anatomy  and  morphol- 
ogy, botanists  have  been  led  to  propose  the  following 

lUThe  fern,  as  we  normally  see  it,  is  an  organism  with,  so  to  speak, 
one  foot  in  the  water,  the  other  on  the  land."  Bower,  F.  O.,  The  origin 
of  a  land  flora,  p.  82. 

2  Scott,  D.  H.     The  Evolution  of  Plants,     p.  18. 


130  HEREDITY  AND   EVOLUTION  IN  PLANTS 

arrangement  of  plant  groups  as  representing  the  general 
course  of  their  evolution  (Table  I) : 

From  what  has  already  been  said,  however,  it  should 
be  understood  that  such  a  table  represents,  not  the  line 
of  evolutionary  advance,  but  the  paths  travelled  by  plants 
in  the  course  of  their  development.  For  example,  it  im- 
plies that  dicotyledons  were  derived  from  monocotyledons, 
pteridophytes  from  bryophytes — hypotheses  which,  from 
other  trustworthy  evidence,  as  stated  above,  now  seem 
untenable. 

TABLE  I . — SEQUENCE  OF  PLANT  GROUPS,  BASED  ON  THE 
MORPHOLOGY  OF  LIVING  FORMS 

Thallophytes  j  Algae — having  chlorophyll, 

(no  archegonia)  \  Fungi — no  chlorophyll. 


f  Bryophytes — no  vascular  system. 


Archegoniates  J  Pteridophytes   | 

(archegonia,  but  no  seeds)    )  Calamophytes  \  vascular  system. 
Lepidophytes    J 

Gymnosperms — no  closed  ovary. 


Spermatophytes 
(seeds) 


Angiosperms — closed  ovary  (pistil). 
Monocotyledons — one-seed  leaf. 
Dicotyledons — two-seed  leaves. 


Again,  the  table  suggests  that  Angiosperms  were  de- 
rived from  Gymnosperms,  and  therefore  appeared  late 
in  the  history  of  plant  life;  but  the  study  of  fossil  plants 
shows  that  they  appeared  in  the  geological  past,  and  were 
dominant  in  the  Tertiary  period,  as  now,  We  are  led, 
therefore  to  proceed  with  caution  in  drawing  inferences 
based  only  upon  a  comparative  study  of  the  structure  of 
forms  now  living. 

104.  Consequences  of  an  Amphibious  Habit  of  Life. — 
The  life  history  of  the  fern  affords  a  concrete  illustration 


THE  EVOLUTION  OF  PLANTS  131 

of  the  consequences  of  a  change  from  an  aquatic  to  an 
amphibious  habit  of  life.  The  gametophyte  is  semi- 
aquatic  in  habit,  and  the  method  of  fertilization  is  purely 
aquatic,  the  sperm  being  unable  to  reach  the  egg  except 
by  swimming  through  free  water.1  But,  when  the  ferti- 
lized egg  began  to  develop  as  a  land  plant,  the  chances  of 
fertilization  by  a  sperm  swimming  in  free  water  became 
increasingly  remote.  The  perpetuation  of  the  species, 
and  the  multiplication  of  individuals  could  be  insured  only 
by  the  formation  of  a  large  number  of  reproductive  bodies 
(spores),  capable  of  distribution  by  wind  in  dry  conditions, 
and  each  able  to  reproduce  its  kind  independently,  without 
fusion  with  another  reproductive  body.  The  larger  the 
number  of  such  spores,  the  greater  the  chances  of  perpetua- 
tion of  the  given  species. 

105.  Consequence  of  Enormous  Spore-production.— 
But  the  formation  of  a  large  number  of  spores  requires  a 
vigorous  plant  body  to  supply  them  with  an  abundance 
of  water  and  nourishment,  and  to  lift  them  up  into  the 
air  where  they  would  stand  a  better  chance  of  distribu- 
tion when  dry.     This  is  accomplished  by  the  sporophyte, 
producing  an  abundance  of  broad,  green  leaves  for  food- 
manufacture,  and  of  roots  for  absorption  of  water  and 
minerals  in  large  quantities.     From  such  considerations  as 
these  the  plant  body  of  the  sporophyte  is  regarded  by  Bower 
and  his  followers  as  produced  by  the  progressive  sterili- 
zation of  tissues  originally  reproductive.     After  the  for- 
mation of  a  vigorous  plant  body,  spores,  produced  in  special 
regions    (sporangia)    could    be   nourished   in   enormous 
numbers. 

106.  Origin  of  Vegetative  Organs. — On  the  basis  of 
Bower's  theory  we  are  to  regard  foliage  leaves  and  branches, 

^eep.  23. 


132 


HEREDITY   AND   EVOLUTION  IN   PLANTS 


either  as  new  formations ,  developed  (by  "enation")  on  some 
primitive  reproductive  axis  like  a  strobilus  or  cone,  or  else 


B 


FIG.  66. — Diagram  to  show  the  increase  in  prominence  of  the  sporo- 
phyte  stage  of  plant  life  from  the  algae  to  the  higher  seed-plants.  Among 
the  thallophytes  both  the  sexual  and  asexual  methods  of  reproduction 
are  represented.  A  illustrates  the  asexual,  wherein  certain  cells  of  the 
plant  divide  into  smaller  cells,  the  zoospores,  which,  without  union  with 
other  cells,  develop  directly  into  new  plants.  B-E  illustrate  the  sexual 
method,  effected  through  an  alternation  of  generations,  wherein  a 
vegetative  stage,  the  sporophyte,  alternates  with  a  reproductive  stage, 
the  gametopyte.  (After  Shimer.) 

as  produced  by  the  sterilization  of  parts  originally  fertile, 
i.e. ,  modifications  of  reproductive  tissues.  The  sporophyte 
has  become  increasingly  well  developed  and  increasingly 
independent,  while  the  gametophyte  has  become  increas- 
ingly simple  and  increasingly  dependent.  The  evolution 
of  plants  has  proceeded  by  the  progressive  development  of 
the  sporophyte,  and  the  gradual  but  steady  regression  of 
the  gametophyte.  This  changing  relationship  is  roughly 
indicated  in  the  following  diagram  (Fig.  67,  and  also  in 
Fig.  66). 
107.  Steps  in  the  Evolution  of  the  Sporophyte.— The 


THE  EVOLUTION  OF  PLANTS 


133 


possible  steps  in  the  evolution  of  the  sporophyte  may, 
on  this  theory,  be  tabulated  as  follows:1 

1.  Sterilization  of  fertile  tissue. 

2.  Localization  of  spore-production  in  sporangia. 

3.  Origination  of  lateral  organs  (leaves),  and  of  roots. 

4.  Development  of  heterospory. 

5.  Introduction    of    fertilization    by    the    pollen-tube 
(siphonogamy) . 

6.  Assumption  of  the  seed-habit. 


FIG.  67. — Diagram  illustrating  the  gradual  change  in  the  relative  promi- 
nence of  the  gametophytic  and  sporophytic  phases  in  the  life-cycle  of 
plants  during  their  evolution  from  the  primitive  algae  (at  the  left)  to  the 
modern  seed-bearing  plants  (at  the  right). 

108.  Second  Hypothesis.— In  a  discussion  of  Bower's 
theory,  Tansley,2  considers  it  "a  priori  in  the  highest 
degree  unlikely  that  so  fundamentally  important  an  organ 
as  the  foliage  leaf  of  the  vascular  plant  appeared  in  descent 
as  an  '  enation '  from  the  surface  of  a  cylindrical  body  of 
different  morphological  nature,"  and  states  that  "there 
is  no  well  established  case  of  any  such  origin  of  an  organ 
of  the  importance  and  with  the  potentialities  of  the  leaf  in 
the  evolutionary  history  of  the  plant  kindgom."  He  also 
calls  attention  to  the  fact  that  the  sporophyte  (sporo- 
gonium)  of  mosses  and  liverworts  has  never  been  known  to 
produce  by  enation  or  otherwise,  any  structure  resembling 

1  Following  F.  O.  Bower. 

2  New  Phytologist  7:177-129.     April  and  May,  1908. 


134  HEREDITY   AND   EVOLUTION   IN   PLANTS 

a  foliage  leaf  or  a  sporophyll,  and  considers  that  it  probably 
as  we  now  know  it,  "  represents  its  highest  capacity  for 
evolution." 

On  the  hypothesis  of  progressive  sterilization  and  ena- 
tion  (strobiloid  theory),  one  would  expect  more  primitive 
sporophytes  to  possess  relatively  small  leaves,  that  is,  to 
be  micr aphyllous,  and  those  with  relatively  large  leaves 
(megaphyllous)  to  be  of  later  evolutionary  development. 
But  there  is  no  fossil  evidence  that  the  microphyllous  fern 
allies  (club-mosses,  horsetails,  sphenophylls)  are  older 
groups  than  the  megaphyllous  true  ferns.  The  suggestion 
is  at  hand,  according  to  Tansley,  that  smaller  leaved  forms 
have  been  derived  from  the  larger  leaved  group  by  reduc- 
tion. The  facts  of  embryology  and  gametophyte  anatomy 
of  the  Lycopods  are  also  interpreted  by  Sykes1  as,  on  the 
whole,  supporting  the  hypothesis  that  the  simpler  Lyco- 
pods are  reduced  forms  and  not  primitive,  the  entire  genus 
Lycopodium  being  regarded  as  formed  by  reduction  from 
some  of  the  larger  fossil  cone-bearing  fern-allies,  such,  for 
example,  as  Lepidodendron  or  one  of  its  near  relatives. 
Miss  Sykes  has  further  suggested  that  the  fossil  genus 
Spencerites  may  represent  the  connecting  link,  between  the 
two  groups. 

It  is  not  possible  nor  essential,  in  a  book  of  this  nature 
and  scope,  to  give  a  detailed  discussion  of  the  evidence  and 
the  literature  bearing  upon  this  and  similar  questions. 
It  is  only  intended  here  to  call  attention  to  the  fact  that 
different  inferences  as  to  the  origin  of  the  leafy  sporo- 
phyte  and  the  broad  course  of  plant  evolution  may  be 

1  Sykes,  M.  G.  Notes  on  the  morphology  of  the  sporangium-bearing 
organs  of  the  Lycopodiaceae.  New  Phytologist.  7:41-60.  Feb.  and 
Mch.,  1908. 


THE   EVOLUTION   OF   PLANTS  135 

logically  deduced  from  the  same  facts,  depending  on  which 
facts  or  classes  of  facts  the  emphasis  is  placed.1 

109.  Homologous  Alternation. — By  the  theory  of  anti- 
thetic alternation  the  leafy  sporophyte  was  derived  from 
some  such  structure  as  the  sporogonium  of  the  Bryo- 
phytes,  the  axis  existing  first,  the  leaves  originating  as  out- 
growths at  its  surface.  There  could  thus  be  no  true 
homology  between  any  of  the  organs  of  the  sporophyte 
and  those  of  the  gametophyte,  however  close  the  super- 
ficial resemblance  might  be.  The  (game tophy tic)  leaves 
of  the  true  mosses,  while  of  like  function  (analogous)  to  the 
(sporophytic)  leaves  of  the  club-mosses,  are  not  the  same 
structural  elements,  i.e.,  are  not  homologous  with  them. 
By  a  contrasting  theory  the  gametophytic  and  sporo- 
phytic stages  were  at  the  first  vegetatively  or  somatically 
equivalent  (except  for  chromosome  number),  as  is  the  case 
now,  for  example,  with  the  red  algae,  Dictyota  and  Poly si- 
phonia,  but,  in  the  course  of  evolution,  the  sexual  phase 
became  more,  and  the  asexual  phase  less,  important  in  other 
forms  (e.g.,  ferns).  This  is  called  the  hypothesis  of 
homologous  alternation,  since  the  vegetative  organs  repre- 
sent the  same  structural  or  morphological  elements. 
According  to  the  antithetic  theory  the  sporophytic  phase 
was  originally  entirely  dependent  on  the  gametophyte 
(as  now,  e.g.,  in  the  Liverworts),  while  according  to  the 
homologous  theory,  the  sporophyte  has  been  free-living 
from  the  start.  By  the  latter  theory,  also,  leaves  did 
not  originate  as  new  formations  at  regions  of  the  axis 
previously  unoccupied  by  lateral  organs  (enation),  but 

1  Those  washing  to  go  more  fully  into  this  question  will  (in  addition  to 
the  article  above  cited)  find  much  of  the  evidence  presented  and  analyzed 
by  Lady  Isabel  Brown  in  a  series  of  five  articles  on  "The  phylogenyand 
inter-relationships  of  the  Pteridophyta,"  in  The  New  Phytologist  for  1908. 
An  extended  bibliography  accompanies  each  article  of  the  series. 


136  HEREDITY   AND   EVOLUTION  IN  PLANTS 

axis  and  foliar  organs  were  both  derived  from  an  ancestral 
thallus,  branching  dichotomously.1 

The  structural  differences  in  the  two  generations  are, 
on  the  basis  of  this  hypothesis,  considered  as  due  almost, 
if  not  entirely,  to  differences  in  environment,  the  main 
factor  being  the  gradual  transition  from  aquatic  to  dry- 
land surroundings.  Where  the  environment  is  uniform 
and  the  same  for  both  generations,  as  for  Dictyota,  the 
gametophyte  and  sporophyte  are  identical  in  external 


FIG.  68. — Diclyola  dicholoma.    Left,  sporogonial  plant;  right,  sperma- 
gonial  (gametophytic)  plant.     (After  W.  D.  Hoyt.) 

organs  and  general  appearance  (Fig.  68).  In  any  event 
the  hypothesis  postulates  a  homology  between  the  various 
organs  of  the  two  generations,  however  much  these  parts 
may  differ  in  external  appearance  as  a  result  of  individual 
variation  and  environmental  influence. 

110.  A  Third  Hypothesis.— Viewing  the  matter  from 
the  standpoint  of  individual  development  (ontogeny)  Lang 
has  developed  the  ontogenetic  hypothesis  of  alternation. 

1  In  a  forked  manner,  resulting  from  the  occurrence  of  two  growing 
points  at  the  tips  of  the  axes, 


THE   EVOLUTION   OF   PLANTS  137 

From  this  point  of  view  two  alternatives  are  recognized : 

1.  Either  the  fertilized  egg  and  the  haploid  spore  are 
potentially  unlike,  and  will  therefore  produce  unlike  plant 
bodies,  even  under  essentially  similar  environment,  or 

2.  Fertilized  eggs  and  spores  are  potentially  alike,  but 
produce  unlike  plant  bodies  as  a  result  of  the  di/erence 
in  the  environment  in  which  they  develop. 

The  ontogenetic  school  accepts  the  latter  alternative 
as  a  working  hypothesis,  and  regards  the  gametophytic 
and  sporophytic  generations  as  essentially  homologous. 
The  degree  of  homology  which  can  actually  be  traced  in 
the  vegetative  structure  of  the  two  generations  may  vary 
from  substantial  identity,  as  in  Dictyota,  to  such  wide 
divergence  that  the  tracing  of  homologies  is  quite  out  of 
the  question.  In  testing  this  hypothesis  a  crucial  ex- 
periment would  be  to  obtain  a  gametophyte  by  artificially 
bringing  a  fertilized  egg  to  mature  development  outside  of 
the  archegonium  and  under  the  environment  in  which 
the  spores  normally  develop;  or  to  obtain  a  sporophyte 
by  causing  a  spore  to  develop  within  the  tissue  of  a  game- 
tophyte, as  the  fertilized  egg  normally  does. 

111.  Hypothetical  Ancestral  Tree. — From  a  compara- 
tive study  of  both  living  and  fossil  forms  some  botanists 
have  been  led  to  infer  the  common  derivation  of  Filicales, 
Equisetales,  and  Lycopodiales  from  the  Hepaticae,  and 
probably  through  some  form  belonging  to  the  Anthocero- 
tales,  somewhat  as  shown  in  the  following  ancestral  "tree" 
(Fig.  69).  It  should  be  clearly  understood  that  this 
tree  does  not  illustrate  known  facts,  but  only  the  hy- 
potheses which  have  been  tentatively  proposed  by  care- 
ful students  on  the  basis  of  known  facts. 

The  evidence  from  fossil  forms  will  be  considered  more 
at  length  in  chapters  XI  and  XII. 


138 


HEREDITY   AND   EVOLUTION   IN  PLANTS 


FILICALENN  STOCK 


FIG.  69. — Hypothetical  genealogical  tree  to  illustrate  the  probable 
affinities  of  the  modern  plant  orders.  This  diagram  is  intended  to  indicate 
that  the  plant  orders  now  existing  are  the  tips,  only,  of  the  branches  of  a 
genealogical  tree,  whose  lower  limbs  and  roots  extend  into  preceding  geo- 
logical periods.  Our  knowledge  is  not  sufficient  to  enable  us  to  connect 
these  branches  with  each  other,  nor  with  the  main  trunk.  The  diagram 
teaches  that  hypothetical  (indicated  by  the  dotted  line)  Anthocerotales 
gave  rise  to  a  now  fossil  Filicalean  stock,  from  which  have  been  derived  all 
the  modern  orders  above  the  mosses  and  liverworts. 


CHAPTER  X 
GEOGRAPHICAL  DISTRIBUTION 

112.  Significance  of  Geographical  Distribution. — From 
the  evidence  of  comparative  anatomy  and  comparative 
life  histories,  and  also  from  the  geological  record  (to  be 
noted  later),  it  has  been  possible  to  determine  the  course 
of  evolution,  in  broad  outlines,  with  reference  to  certain 
of  the  larger  groups  of  plants.  As  noted  above,  we  may 
learn  that,  in  all  probability,  ferns  preceded  gymnosperms, 
and  gymnosperms  preceded  angiosperms;  but  within  these 
various  groups,  and  for  living  forms,  the  problem  becomes 
increasingly  difficult.  For  example,  how  shall  we  deter- 
mine whether  the  family  represented  by  the  bracken  fern 
(Polypodiacece)  is  more  ancient  or  more  modern  than  the 
royal-fern  family  (Osmundacece)?  Is  the  maiden-hair 
tree  (Gink go)  a  younger  or  an  older  species  than  the  pine 
and  the  hemlock?  Did  herbs  precede  trees  in  the  evolu- 
tion of  Angiosperms,  or  vice  versa?  This  question  of  the 
relative  ages  of  living  groups  is  greatly  illuminated  by  the 
evidence  afforded  by  the  facts  of  geographical  distribution 
of  fossil  and  living  forms. 

Darwin  spoke  of  geographical  distribution  as  the 
"almost  keystone  of  the  laws  of  creation,"1  and  one  does 
not  need  to  pursue  the  study  of  that  subject  far  to  under- 
stand the  truth  of  his  statement.  Before  the  diffusion  of 

1  The  interested  reader  will  wish  to  consult  those  two  remarkable  chapters 
(XII  and  XIII)  of  volume  two  of  the  Origin  of  Species. 
139 


140 


HEREDITY   AND   EVOLUTION   IN  PLANTS 


Darwin's  teaching,  which  freed  men's  minds  from  the 
shackles  of  preconceived  notions,  founded,  not  on  observa- 
tion of  facts,  but  on  a  more  or  less  blind  acceptance  of 
current  theological  suppositions,  or  on  the  teachings  of 
ancient  writers,  the  facts  of  distribution  had  a  far  different 
significance  than  they  were  seen  to  possess  when  men  began 
to  interpret  the  present  state  of  nature  as  being  the  result 


FIG.  70. — Alexander  von  Humboldt  (1769-1859).     Founder  of  the 
science  of  plant  geography. 

of  the  operation  of  natural  causes;  by  most  students  they 
had  been  regarded  as  so  much  information,  like  the  matter 
in  a  guide  book,  but  pointed  the  way  to  no  larger  conception 
or  generalization,  so  far  as  historical  evolution  was  con- 
cerned. To  find  giant  redwood  trees  exclusively  in  Cali- 
fornia meant  nothing,  except  that  they  were  created  there 
and  nowhere  else,  and  had  never  spread;  to  find  the  bracken 


GEOGRAPHICAL  DISTRIBUTION  141 

fern  of  almost  universal  occurrence,  in  both  temperate 
and  torrid  latitudes,  eastern  and  western  hemispheres, 
could  be  easily  explained  on  the  supposition  that  it  had 
gradually  spread  from  the  center  of  distribution  where 
it  was  created,  or  on  the  theory  that  it  had  been  indepen- 
dently "created"  in  many  different  localities. 

The  idea  that  the  same  species  was  " created"  indepen- 
dently in  different  localities,  from  which  it  might  spread, 
was  taught  by  Gmelin  as  early  as  1747.  It  is  often 
referred  to  as  Schouw's  hypothesis,  from  the  Danish 
botanist  who  elaborated  and  urged  it  in  the  first  part  of 
the  nineteenth  century.1  Reasoning  from  the  facts  of 
discontinuous  distribution  (to  be  noted  in  following  para- 
graphs) Schouw  argued  for  the  hypothesis  of  the  multiple 
origin  of  species,  that  is,  that  there  were  originally  many 
primary  individuals.  The  existing  vegetation  of  the 
globe  was  not  created  at  once,  argued  Schouw,  but  by 
degrees,  since  the  surface  of  the  earth  has  only  gradually 
become  fitted  for  the  growth  of  plants,  and  moreover 
certain  plants  (e.g.,  parasites)  depend  upon  the  existence 
of  others,  and  therefore  the  latter  must  have  previously 
existed.2  The  hypothesis  of  multiple  origin  was  also,  at 

1  Schouw,  J.  F.     Desedibus  plantarum  originariis,  Hauniae,  1816.     His 
memoir  On  the  origin  of  plants  was  published  in  Danish  in  1847,  and  the 
English  translation  by  N.  Wallich  was  published  in  Hooker's  Journal  of 
Botany,  2:321-326,373-377.    London,  1850;  and  Ibid,  3:  11-14, 1851. 

2  This  is  an  interesting  illustration  of  how  the  same  kind  of  evidence  may 
lead  one  student  toward  the  truth  and  another  toward  error.     Schouw  was 
proposing  the  ideas  here  set  forth  at  the  same  time  that  Darwin  was 
elaborating  his  theory  of  natural  selection,  and  only  twelve  years  before 
the  appearance  of  the  Origin.     Raising  the  question  as  to  whether  new 
species  continue  to  be  created,  or  whether  the  existing  vegetable  kingdom 
has  been  finally  completed,  he  argues  that,  "The  most  rational  mode  for 
accounting  for  new  species  being  possibly  created,  seems  to  be  by  suppos- 


142  HEREDITY  AND   EVOLUTION  IN   PLANTS 

first,  adopted  by  Alphonse  DeCandolle,  but  finally  aban- 
doned by  him  in  his  Geographic  Botanique  Raison&e  (1855). 
About  the  middle  of  the  last  century  Agassiz  was  urging 
his  autochthonous1  hypothesis,  namely,  that  each  species 
originated  where  it  is  now  found  (indigenous),  covering 
from  the  first  as  large  an  area  as  at  present.  This  hy- 
pothesis, if  true,  would,  as  Gray  pointed  out,  "remove  the 
whole  question  out  of  the  field  of  inductive  science." 
There  would  be  no  incentive  to  study  the  question  of 
geographical  distribution,  and  little  of  value  could  result 
from  such  an  investigation.  Both  Schouw's  and  Agassiz's 
ideas  have  long  since  been  abandoned.  It  is  no  longer 
considered  a  matter  of  hypothesis  or  theory,  but  of  well 
established  fact,  that  most  of  the  existing  species  are  im- 
measurably older  than  the  present  configuration  of  the 
continents;  in  fact  many  genera  and  families  of  Angio- 
sperms  of  the  present  land  flora  were  clearly  defined  as 
early  as  the  Tertiary  period,  and  have  undergone  little 
change  since  that  remote  time.  • 

113.  Means  of  Dispersal. — The  question  of  the  means 
of  dispersal  of  the  seeds  and  spores  of  plants  is  a  large  one, 
and  a  voluminous  literature  exists  on  the  subject.  This 
is  not  the  place  to  go  extensively  into  the  matter,  but  a  dis- 

ing  that  a  change  of  climate  or  soil  produces  a  corresponding  change  in 
the  character  of  its  plants;  or  that  some  casual  difference  in  the  character 
of  the  type  of  any  given  plant,  may  have  become  permanent  by  its  being 
isolated.  It  is  in  this  way  that  constant  varieties  have  arisen,  which  may 
sometimes  even  have  become  real  species,  but  on  all  these  occasions  it  is 
culture  that  has  been  the  cause;  as  far  as  I  know,  we  possess  no  facts  to 
prove  that  natural  causes  have  produced  this  effect."  Schouw  also 
reached  the  erroneous  conclusion  that  the  present  flora  was  probably  not 
derived  from  the  plants  of  preceding  geological  periods. 

1  Autochthon,  from  the  Greek  auras,  self  +x6uv  land,  meaning  from  th 
land  itself. 


GEOGRAPHICAL  DISTRIBUTION  143 

cussion  of  geographical  distribution  requires  a  clear  under- 
standing of  certain  points  which  may  be  briefly  alluded 
to  here.  The  situation  with  plants  is,  of  course,  quite 
different  than  that  with  animals.  With  the  advance  of  a 
continental  ice  sheet,  for  example,  animals  may  actively 
retreat,  by  their  own  locomotion.  There  are  exceptions 
to  this  method  of  animal  dispersal.  Insects  and  small 
birds  may  be  blown  by  the  wind  over  considerable  dis- 
tances, and  insect  eggs,  larvae,  and  cocoons  may  be  trans- 
ported in  the  soil  about  the  roots  of  floating  uprooted  trees 
and  otherwise;  and  instances  are  on  record  of  animals 
being  carried  as  passengers  on  floating  objects,  notably, 
according  to  Semon,  in  the  Malay  Archipelago;  snakes 
and  crocodiles  are  known  to  have  drifted  in  this  way  to 
the  shores  of  the  Cocos  or  Keeling  Islands,  a  group  of 
coral  atolls  in  the  Indian  Ocean,  about  700  miles  south- 
west of  Java,  the  nearest  land. 

But  plants,  at  all  times  and  under  all  circumstances, 
are  wholly  dependent  on  being  carried  passively  by  exter- 
nal agencies.  The  chief  means  of  seed  dispersal  are  the 
wind,  streams  and  ocean  currents,  and  animals — particularly 
birds.  For  distribution  over  great  distances  it  is  of  im- 
portance to  ^consider^  chiefly  wind,  ocean  currents,  and 
birds.1 

114.  Dispersal^by^Wind. — DeCandolle,  the  great 
Swiss  student  of  plant  geography,  regarded  the  wind  as 
"the  most  general  and  ordinary  cause  of  the  distribution 
of  species  over  the  entire  surface  of  a  country,"  but  re- 
jected it  as  a  means  of  dispersal  over  even  narrow  arms  of 

1  The  distribution  of  seeds  in  connection  with  commercial  shipments  is 
interesting,  but  not  essential  to  our  present  purpose.  The  word  "seeds" 
is  used  above  to  designate  all  reproductive  bodies,  including  fruits,  spores, 
and  vegetative  reproductive  bodies,  such  as  gemmae,  bulbils,  etc. 


144  HEREDITY   AND   EVOLUTION   IN   PLANTS 

the  sea.  "I  have  never  heard,"  he  said,1  "of  a  single  seed 
carried  from  England  to  France,  nor  from  Ireland  to 
England  by  the  agency  of  the  west  winds,  although  they 
are  so  intense  and  so  frequent  in  those  countries.  I  do 
not  believe  it  has  ever  been  demonstrated  that  seeds  have 


FIG.  71. — Alphonse  De  Candolle  (1806-1893).  Noted  Swiss  botanist 
and  student  of  plant  geography.  Author  of  Geographic  Botanique 
Raisonee.  (From  Ada  Horli  Bergiani.) 

fallen  in  Sardinia  from  Africa,  in  Corsica  from  Sardinia, 
nor  from  Corsica  to  the  coast  of  Genoa  and  Nice,  although 
the  south  winds  are  there  very  violent."  Other  students 
have  reached  the  same  conclusion  on  similar  negative 
evidence.  It  has  also  been  argued,  on  general  grounds, 

1  Geographic  Botanique  Raisonee  2: 614.     Geneva,  1855. 


GEOGRAPHICAL   DISTRIBUTION  145 

that  the  rate  of  fall  of  various  seeds  in  air  is  such  that  they 
would  have  to  be  carried  to  improbable  heights  by  the 
wind  in  order  to  travel  for  very  great  distances  before 
falling  to  the  ground. 

But  no  amount  of  negative  evidence  is  conclusive  in  the 
face  of  even  one  firmly  established  bit  of  positive  evidence, 
and  the  positive  evidence  is  not  only  more  conclusive  but 
more  voluminous  than  the  negative.  Seeds  of  the  pitcher 
plant,  Nepenthes  ampullaria,  are  known  to  have  been 
transported  from  Ceylon  to  the  Seychelles,  a  distance  of 
1500  miles,  and  Engler  calculated  that,  out  of  a  total  of 
about  675  species  in  Hawaii,  140  ferns  and  other  spore- 
bearing  plants,  and  14  angiosperms  were  quite  certainly 
transported  thither  by  wind.  In  fact,  a  large  percentage 
of  the  vegetation  of  isolated  oceanic  islands  is  of  plants 
whose  seeds  could  hardly  have  been  transported  there  in 
a  viable  condition  except  by  winds.1 

As  Warming  has  noted,  there  is  not  an  oceanic  island 
destitute  of  plant  life,  though  many  of  them  are  separated 
from  the  present  mainland  by  hundreds  and  even  thousands 
of  miles  of  salt  water,  and  have  never,  in  all  probability, 
been  connected  with  any  continent;  all  their  vegetation, 
therefore,  must  have  been  transported  thither  by  some 
agency.  In  1901  there  fell  in  Switzerland  large  quantities 
of  dust  which  is  said  to  have  undoubtedly  come  from 
Africa.  If  this  were  possible  it  is  certainly  not  improb- 
able that  light  seeds  of  various  species  might  be  trans- 
ported very  long  distances  in  a  similar  manner.  The 

1  Even  small  animals,  and  especially  insects,  are  known  to  be  trans- 
ported to  considerable  distances  by  the  wind.  During  his  voyage  on 
the  exploring  ship  Beagle  (1832-1836),  Darwin  observed  spiders,  buoyed 
up  by  their  webs,  being  wafted  over  the  vessel  by  the  wind  as  far  as 
60  miles  from  land. 


146  HEREDITY   AND   EVOLUTION  IN   PLANTS 

wind  is  known  to  be  a  factor  in  plant-distribution  in  the 
West  Indies.  Thus,  for  example,  previous  to  1899,  the 
sedge,  Fimbristylis  spathacea  Roth,  was  not  known  on 
Great  Bahama  island,  of  the  West  Indies.  After  the  hur- 
ricane of  August  13,  1899  this  sedge  appeared  in  clearings, 
and  "soon  spread  as  a  troublesome  weed  through  culti- 
vated lands,  killing  out  pasture  grass  in  places;  it  had 
therefore  come  to  be  called  'Hurricane  Grass."1 

In  August,  1883,  the  island  of  Krakatoa,  west  of  Java 
in  the  Sunda  Strait,  experienced  a  terrific  volcanic  erup- 
tion, which  completely  destroyed  every  vestage  of  its 
vegetation,  converting  the  green  island  into  a  desolate 
desert.  Within  three  years  thereafter  Treub  found  grow- 
ing there  six  algae  and  26  vascular  plants,  including  n 
ferns  and  15  spermatophytes.  '  A  little  over  ten  years 
after  Treub's  visit  Penzig  found  62  vascular  plants,  of 
which  60  per  cent,  had  been  brought  by  ocean  currents, 
32  per  cent,  by  wind,  and  7  per  cent,  by  fruit-eating  birds. 
Within  twenty-five  years  from  this  eruption  the  island  was 
again  green  with  forest  growth  and  other  vegetation,  and 
in  1906  a  party  of  botanists  confined  their  collecting  to  a 
narrow  zone  of  forest  near  the  shore  because  of  the  diffi- 
culty of  "cutting  a  way  through  the  dense  growth  of  tall 
grasses"  between  the  shore  and  the  volcanic  cone  in  the 
center  of  the  island.  Among  the  means  of  transportation 
of  plant  life  to  Krakatoa,  the  wind  is  regarded  by  Ernst  as 
a  factor  of  exceptional  importance.  Up  to  1906,  as  cal- 
culated by  him,  39-72  per  cent,  of  the  total  number  of 
phanerogams  on  the  Krakatoa  group  were  brought  by  ocean 
currents.  Ten  to  19  per  cent,  of  the  entire  flora  by  birds,  and 
16-30  per  cent,  by  air-currents.  Beccari  found  the  same 

'Britton  &  Millspaugh.     Bahama  Flora.,  p.  51.     Unpublished. 


GEOGRAPHICAL   DISTRIBUTION  147 

species  on  widely  separated  mountain  tops  in  the  Malay 
Archipelago  where  wind  (particularly  the  west  monsoon) 
is  the  only  agent  of  dispersal  that  may  reasonably  be  as- 
signed. The  seeds  of  many  plants  are  as  light  as  dust 
particles,  and  it  has  been  calculated  that  nearly  850,000,- 
ooo  tons  of  dust  are  transported  as  far  as  1,440  miles  a 
year  in  the  western  United  States.1  In  the  light  of  this 
information  it  is  not  difficult  to  understand  how  seeds  of 
Nepenthes  phyllamorpha,  that  weight  only  0.000035  gram, 
seeds  of  Rhododendron  verticillatum  and  of  Dendrobium 
altenuahim,  that  weigh  0.000028  gram  and  0.00000565 
gram  respectively,  can  be  transported  many  miles,  re- 
sulting in  a  geographical  distribution  of  those  (and  various 
other)  species,  on  the  mountain  tops  of  oceanic  islands  that 
are  miles  apart. 

James  Small  has  carried  out  a  series  of  painstaking 
experiments  on  the  transportation  of  the  seeds  of  various 
plants  by  artificially  produced  air  currents.  Among  many 
valuable  results  of  these  experiments,  he  determined  that 
for  the  seeds  of  the  dandelion,  "so  long  as  the  relative  hu- 
midity of  the  air  remains  above  0.77  per  cent,  and  so  long 
as  the  fruit  does  not  encounter  an  obstacle,  a  horizontal 
wind  of  1.97  miles  per  hour  is  sufficient  for  its  dispersal  to 
any  distance.  If  the  air  becomes  moist  the  pappus  closes  up 
and  the  fruit  falls  rapidly."  Small  further  concludes  that 
the  ordinary  pappose  fruit  of  the  Compositae,  under  the 
proper  meteorological  conditions,  can  be  blown  many  hun- 
dreds of  miles  over  land  and  sea,  and  "that  hypothetical 
land  bridges  are  not  necessary  to  explain  the  present  dis- 
tribution of  the  Compositae,  so  that  we  can  take  the  world 

1  Cited  by  James  Small  (NewPhytologist  17:  226.  1918)  from  Evans,  J.  W. 
The  wearing  down  of  rocks, Pt.II,  Proc.Geol.  Assoc.  25,  Pt/4:22g.  1914. 


148  HEREDITY   AND   EVOLUTION   IN  PLANTS 

as  it  is  without  raising  and  sinking  continents,  as  Darwin 
says,  'in  a  quite  reckless  manner.'  This  latter  is  an  impor- 
tant point,  as  the  Compositae  are  almost  certainly  of  such 
recent  origin  that  the  possibility  of  land  bridges  is  in  many 
cases  quite  out  of  the  question."  In  fact,  Small  contends 
that  a  "rational  study  of  the  history  of  the  Composite, 
their  migrations  and  colonizations,  their  paths  of  travel 
and  regions  of  concentration,"  is  not  possible  without  a 
correct  understanding  of  the  conditions  of  wind  dispersal. 
The  occurrence  of  a  species  of  Senecio  (a  pappose- 
fruited  Composite)  on  the  Falkland  Islands,  300  miles 
from  the  nearest  land,  and  of  another  species  on  St. 
Helena  and  on  Prince's  Island,  nearly  1500  miles  from  the 
nearest  land,  are  attributed  by  Small  respectively  to  the 
westerly  and  the  south-east  trade  winds.  The  distri- 
bution of  the  family,  as  worked  out  in  detail  by  Small1 
affords  an  instructive  illustration  of  how  geographical 
distribution  affords  new  evidence  and  confirms  other 
evidence  as  to  the  relative  ages  of  various  related  groups 
of  plants,  and  as  to  the  fact  and  course  of  evolution  within 
a  given  plant  family.  The  immense  genus,  Senecio, 
for  example,  according  to  Small,  comprising  over  2300 
species,  is  of  very  wide  distribution,  being  marked  by  a 
concentration  at  high  altitudes,  which  is  not  surprising  in  a 
wind-distributed  group.2  Some  of  the  species  are  wide- 
spread, and  some  are  local,  and  the  group  is  characterized 
by  its  ready  response  to  the  influence  of  environment; 
to  this  is  attributed,  in  large  part,  its  great  morphological 
variation.  No  species  covers  the  range  of  the  genus. 

1  Small,   James.     The    origin  and   development  of  the   Compositae. 
Chapter  X.     New  Phytologist  18  :  1-35.     Jan.  and  Feb.,  1919. 

2  It  has  been  calculated  by  Ball  that  25-30  per  cent,  of  the  flora  of  the 
higher  Andes  are  Composites. 


GEOGRAPHICAL   DISTRIBUTION  149 

Small  has  shown  that  the  local  species  have  regions  of 
concentration  along  the  paths  of  migration  of  the  wide- 
spread species,  and  that  they  are  most  abundant  "along 
the  ridge  which  extends  around  the  Pacific  and  Indian 
Oceans  from  Tierra  del  Fuego  to  South  Africa."  The 
paths  of  migration  are  chiefly  coextensive  with  the  alti- 
tude of  3,000  ft.,  or  higher,  and  all  the  facts  point  to  the 
Andes  of  Bolivia  as  the  probable  (hypothetical)  center  of 
distribution  for  the  genus,  whence  it  has  rapidly  spread 
"along  the  unwooded  regions  of  the  mountain  ranges  of  the 
world."  This  world-wide  distribution,  and  the  posses- 
sion of  pappose  fruits  which  would  make  possible  a  wide 
distribution  in  a  relatively  short  period  of  time  (geologi- 
cally speaking),  all  point  (as  do  the  facts  of  its  morphology) 
to  the  comparative  youth  of  the  group;  while  its  marked 
tendency  to  variation,  its  success  in  the  struggle  for  exis- 
tence (as  may  be  noted  everywhere),  and  finally  the 
existence  of  innumerable  local  species,  with  centers  of 
distribution  along  the  paths  of  migration  of  the  genus  as  a 
whole,  are  just  the  facts  which  one  would  expect  to  find 
on  the  basis  of  the  theory  of  evolution. 

115.  Dispersal  by  Water  and  Birds. — Space  at  our 
disposal  will  permit  of  only  a  passing  reference  to  seed- 
dispersal  by  water  and  birds.  In  order  to  be  carried  for 
long  distances  by  water,  seeds  and  spores  must^be  able  to 
undergo  prolonged  soaking  in  water,  and  in  the  case  of 
ocean  currents,  in  salt  water.  Many  species  of  the  new 
strand  flora  of  Krakatoa  were  certainly  brought  many 
miles  by  ocean  currents,  and  Guppy,  who  made  a  study 
of  "Plants,  seeds,  and  currents  in  West  Indies  and  Azores," 1 
cites  the  case  of  a  ragweed  (Ambrosia  crithmifolia]  whose 

1  Guppy,  H.  B.     London,  1917. 


150  HEREDITY   AND   EVOLUTION   IN   PLANTS 

seeds  were  dispersed  on  floating  logs;  and  the  small  seeds 
of  several  species  were  safely  transported  in  the  crevices 
and  holes  made  in  small  stems  and  branches  by  worms  and 
molluscs.  Other  seeds  were  floated  on  blocks  of  pumice. 

In  Hawaii,  while  nearly  85  per  cent,  of  the  spermato- 
phyte  flora  is  endemic  (see  p.  165),  about  70  per  cent, 
of  the  species  of  the  coastal  zone  are  introduced.  This 
is  in  marked  contrast  to  the  general  rule  for  oceanic 
islands,  whose  littoral  floras,  as  might  be  expected,  are 
predominantly  cosmopolitan.  In  this  particular  case 
MacCaughey  attributes  very  great  importance  to  ocean 
currents  as  agents  of  dispersal.  The  natives  of  these 
islands,  at  the  time  of  their  discovery,  are  reported  to  have 
had  large  canoes  hewn  from  tree  trunks  of  the  Douglas 
spruce,  which  could  have  come  only  from  the  north- 
west shores  of  North  America;  and  considerable  numbers 
of  tree  trunks  and  large  branches  are  brought  from  the 
same  coasts  to  Hawaii  each  year.  Ocean  currents  also 
bring  annually  large  quantities  of  plant  material  to  the 
coast  of  other  oceanic  islands.  Tansly  and  Fritsch  have 
noted  large  numbers  of  young  seedlings  and  germinating 
seeds  in  drift  material  on  the  coast  of  Ceylon,  and  Moseley 
observed  many  living  plants  in  the  coastal  drift  of  the 
Moluccas,  including  trees,  palms,  epiphytic  orchids,  and 
large  quantities  of  fruits  containing  viable  seeds. 

Seeds  are  carried  by  birds  in  mud  adhering  to  their 
feet,  lodged  in  their  feathers,  and  in  the  alimentary  canal. 
In  mud  adhering  to  the  feet  of  a  partridge  Darwin  found 
82  seeds  that  germinated.  Wallace  is  authority  for  the 
statement  that  "all  the  trees  and  shrubs  in  the  Azores 
bear  berries  or  small  fruits  which  are  eaten  by  birds; 
while  all  those  which  bear  larger  fruits,  or  are  eaten 


GEOGRAPHICAL   DISTRIBUTION  151 

chiefly  by  mammals — such  as  oaks,  beeches,  hazels, 
crabs,  etc. — are  entirely  wanting.1  It  has  been  suggested 
by  both  Guppy  and  Schimper  that  the  wide  distribution 
of  fig  trees  in  oceanic  islands  (e.g.  Malay  and  Solomon 
Archipelagos)  is  due  to  their  fruit  being  eaten  by  doves 
and  other  birds  capable  of  sustained  flight.  The  pro- 
digious powers  of  flight  of  some  of  the  migratory  birds 
would  make  them,  theoretically  at  least,  most  efficient 
agents  of  seed  dessemination  over  wide  areas.  The  scarlet 
tanager,  for  example,  breeds  in  the  eastern  United  States 
from  Oklahoma  to  the  mountains  of  North  Carolina,  and 
north  to  New  Brunswick  and  Saskatchewan.  At  the 
close  of  summer  the  birds  migrate  south,  passing  from 
the  Gulf  coast  of  the  United  States  to  and  along  Central 
America  to  the  west  tropical  coast  of  South  America. 
The  arctic  tern  nests  during  the  northern  summer  along  the 
northeast  arctic  coast  of  North  America  and  the  southwest 
coast  of  Greenland,  but  passes  the  northern  winter  within 
the  Antarctic  Circle,  11,000  miles  away.  Passing  as  it 
does  through  regions  of  similar  climate  in  the  northern 
and  southern  hemispheres,  it  would  theoretically  be 
possible  for  spores  and  light  seeds  to  be  carried  to  con- 
genial habitats  on  both  sides  of  the  equator.  The 
American  golden  plover  breeds  in  summer  along  "the 
northern  coast  of  Canada,  the  parallel  of  70°  north  latitude 
passing  approximately  through  the  center  of  its  breeding 
range.  In  early  fall  the  birds  migrate  to  Labrador, 
thence  to  Nova  Scotia,  and  thence,  after  a  few  weeks, 
in  a  straight  flight  of  2,400  miles  to  the  north  coast  of 
South  America.  From  their  landing  point  the  birds 
pass  to  Argentina  where  they  pass  the  northern  winter, 

1  Wallace,  A.  R.,  Darwinism,  3d  edition,  p.  361. 


152  HEREDITY   AND    EVOLUTION  IN  PLANTS 

returning  the  following  spring  to  the  Arctic  coast,  but 
by  an  entirely  different  rout,  passing  over  Central  America, 
the  Gulf  of  Mexico,  and  across  central  North  America. 

The  Pacific  golden  plover  nests  along  the  arctic  shore 
of  Eastern  Siberia  and  the  western  coast  of  Alaska, 
but  winters  in  southeastern  Asia,  eastern  Australia 
and  generally  in  the  islands  of  Oceanica,  the  winter  home 
having  an  east-west  range  of  about  10,000  miles.  The 
journey  from  Alaska  to  Hawaii,  a  distance  of  some  3,000 
miles,  is  made  in  a  single  flight.1  Whether  seed  dispersal 
is  actually  accomplished  by  any  of  the  above  long  distance 
travelers  is  not  definitely  known  to  the  writer;  but  the 
flights  are  accomplished  in  a  comparatively  brief  period, 
and  it  seems  not  unreasonable,  from  what  we  actually 
know  of  seed-transportation  by  birds,  that  lighter  and 
more  resistant  seeds  and  spores  of  plants  may  be  thus, 
transported,  concealed  in  the  plumage  of  the  birds,  or 
otherwise,  and  between  stations  where  no  other  known 
agent  of  dispersal  would  appear  to  be  adequate. 

In  1911  a  violent  eruption  of  the  Taal  Volcano,  on 
Volcano  Island,  Luzon,  Philippine  Islands,  "annihilated" 
(as  Maso  described  it)  every  vestage  of  vegetation  on  the 
island.  The  destruction  was  caused  by  superheated 
steam,  and  by  the  deposit  of  a  layer  of  fine  "mud," 
which  fell  like  rain,  and  carried  with  it  large  quantities 
of  sulphur  dioxide  and  possibly  other  substances  fatal 
to  plant  life.  In  a  study  of  the  revegetation  of  the  island, 
made  six  years  after  the  eruption,  Brown,  Merrill,  and 
Yates  found  evidence  that  birds  were  the  most  important 

1  For  the  above  data  on  bird  migrations  the  author  is  indebted  to  the 
article  on  "Our  greatest  travelers,"  by  Wells  W.  Cooke.  Nat.  Geographic 
Mag.  22  :  346-365.  April,  1911. 


GEOGRAPHICAL  DISTRIBUTION  153 

agents  of  transportation  of  seeds  for  the  new  growth, 
about  54  per  cent,  of  a  total  of  157  species  having  seeds 
adapted  to  dispersal  by  birds,  only  21  per  cent,  adapted 
to  wind  dispersal,  and  only  9  per  cent,  apparently  brought 
by  currents  of  water.  Of  course,  the  distance  from 
Volcano  Island  to  the  nearest  uninjured  vegetation  was 
short,  and  the  various  agents  of  dispersal  would,  no  doubt, 
assume  a  different  relative  importance  for  greater  dis- 
tances, as  they  did,  for  example,  in  the  case  of  Krakatoa, 
noted  above.1 

116.  Struggle  for  Existence  a  Factor.— DeCandolle 
early  contended  that  it  was  not  sufficient  that  one  or 
even  a  few  seeds  be  carried  to  a  country  already  well 
covered  with  vegetation  in  order  for  the  new  arrivals  to 
become  established,  but  that  a  very  large  number  of 
vigorous  seeds/must  be  introduced  to  insure  success  in 
the  struggle  for  existence  with  the  native  plants.  Atten- 
tion has  been  called  in  a  preceding  chapter  (pp.  94-96), 
however,  to  the  enormous  rate  of  propagation  of  plants 
and  animals,  which  proceeds  in  geometrical  progression; 
so  that  if  we  allow  a  sufficient  time  period,  and  postulate 
a  species  suited  to  the  climate  and  soil  of  its  newly  found 
home,  we  may  expect  a  large  degree  of  success  in  its  be- 

1  In  contradiction  to  the  above  statements  of  fact  and  logical  inference, 
there  should  be  noted  here  Warming's  quotation  (Botany  of  the  Faeroes), 
p.  676.  London,  1901-1908)  from  the  Danish  Zoologist,  H.  Winge,  of  the 
Zoological  Museum,  Copenhagen,  who  stated  in  a  letter  to  Warming  that 
he  had  carefully  examined  thousands  of  migratory  birds  picked  up  dead  at 
Danish  lighthouses,  and  had  never  found  any  seeds  adhering  to  the  feathers 
beaks,  or  feet.  Dried  mud  was  found  "fairly  often,"  but  there  were  ad- 
hering to  it  no  seeds  large  enough  to  be  seen  with  the  naked  eye  or  the 
hand-lens.  Moreover  the  stomachs  of  migrating  birds  were  always  found 
to  be  practically  empty,  indicating  that  migrating  birds  travel  on  empty 
stomachs.  See,  however,  p.  164,  infra. 


154  HEREDITY   AND   EVOLUTION   IN   PLANTS 

coming  established,  at  least  in  limited  area  and  numbers, 
even  though  the  number  of  seeds  introduced  was  compara- 
tively small.  Farmers  in  America  can  bear  emphatic 
but  sad  testimony  to  their  practical  helplessness  to  com- 
bat successfully  the  spread  through  the  hay  fields  of  the 
hated  daisy  or  white-weed  (Chrysanthemum  leucanthemum) , 
or  the  still  more  dreaded  Devil's  paint  brush  (Hieracium 
aurantiacum).  Both  of  these  species  are  now  common 
weeds  in  America,  though  introduced  from  Europe,  the 
former  almost,  and  the  latter  quite  within  the  memory  of 
men  now  living.  Wallace  has  stated  that,  if  a  million 
seeds  were  brought  to  the  British  Isles  by  wind  in  one  year, 
there  would  be  only  ten  seeds  to  a  square  mile.  "The 
observation  of  a  life  time  might  never  detect  one,  yet  a 
hundredth  part  of  this  number  would  serve  in  a  few  cen- 
turies to  stock  an  island  like  Britain  with  a  great  variety 
of  continental  plants."  When  we  recall  the  enormous 
mortality  of  seeds  and  seedlings,  such  facts  as  these  enable 
us  to  appreciate  the  importance  to  a  species  of  an  abundance 
of  spore  and  seed  production,  as,  for  example,  in  dandelions 
and  other  composites,  in  ferns,  and  indeed  in  most  plants. 
117.  Types  of  Distribution. — There  are  two  broad 
types  of  geographical  distribution;  continuous,  as  in  the 
case  of  the ;  bracken  fern  (Pteris  acquilina);  and  discon- 
tinuous, as  in  the  case  of  the  Osmunda  family,  where  a 
given  species  is  found  in  widely  separated  localities,  but 
not  in  the  intervening  regions.  Osmunda  regalis  (the 
Royal  Fern),  for  example,  is  known  from  eastern  North 
America,  central  and  northern  Asia,  and  Europe;  Os- 
munda Japonica  from  central  and  northern  Asia  and  Japan 
and  the  cinnamon  fern  (Osmunda  cinnamomea)  only  from 
eastern  North  America  and  Japan.  The  genus  Diervilla, 


GEOGRAPHICAL   DISTRIBUTION 


155 


of  the  Honeysuckle  family,  is  represented  in  the  eastern 
United  States  and  Canada  by  the  bush-honeysuckle 
(Diervilla  Lonicera),  and  in  the  mountains  of  the  southern 
States  by  D.  sessilifolia  and  D.  rivularis;  it  is  not  found 
elsewhere  except  in  eastern  Asia,  where  it  is  represented 
by  the  shrubs  commonly  cultivated  in  temperate  America 
under  the  name  Weigela. 

In  the  herbarium  of  the  Brooklyn  Botanic  Garden  are 
two  specimens  of  the  cloud-berry,  or  mountain  bramble 
(Rubus  chamcemorus) ,  collected  in  a  bog  near  Montauk 


FIG.    72. — Map    showing    the  geographical  distribution  of    the  skunk- 
cabbage,  Symplocarpus  fcetidus.     (After  M.  L.  Fernald.) 

Point,  Long  Island,  by  Dr.  William  C.  Braislin,  in  1908. 
This  is  an  arctic  and  sub-arctic  bog  plant,  ranging  from 
Labrador  and  Newfoundland  to  New  Hampshire,  British 
Columbia,  and  Alaska;  also  in  Europe  and  Asia.  It  was 
found  on  the  Peary  arctic  expedition  as  far  north  as  Lat. 
64°  15'  north.  Its  discovery  as  noted  above  was  unex- 
pected, and  affords  an  interesting  example  of  discontinuity 
of  distribution.  Another  striking  illustration  is  the  "curly 
grass"  fern  (Schizaa  pusilla) ,  of  the  Polypodiacese,  found  in 
Nova  Scotia  and  Newfoundland,  and  in  the  pine  barrens  of 
southern  New  Jersey,  but  not  known  to  occur  between 


156  HEREDITY   AND   EVOLUTION   IN  PLANTS 

these  two  regions.  The  skunk-cabbage  (Symplocarpus 
fcetidus,  Fig.  72),  species  of  Magnolia,  Hydrangea, 
Hamamelis  (witch-hazel),  Liquidambar  (sweet-gum),  Ara- 
lia  (ginseng),  Eupatorium,  Onoclea  (sensitive  fern),  Ly co- 
podium  (L.  lucidulum),  and  scores  if  not  hundreds  of  other 
species,  have  a  similar  type  of  distribution. 

Similarity  of  Floras  of  Eastern  Asia  and  Eastern 
North  America. — The  similarity  in  the  floras  of  eastern 
North  America  and  eastern  Asia  and  Japan  was  first 
pointed  out  by  Asa  Gray,1  in  1859,  on  the  basis  of  his 
study  of  the  plants  collected  in  Japan  in  1855,  by  Charles 
Wright,  botanist  of  the  U.S.  North  Pacific  exploring  ex- 
pedition. Of  580  Japanese  plants  of  this  collection 
Gray  found  only  0.37  per  cent,  having  representatives  in 
western  North  America,  while  0.61  per  cent,  has  repre- 
sentatives in  eastern  North  America;  for  identical  species 
the  corresponding  percentages  were  0.27  per  cent,  and  0.23 
per  cent.  Of  56  Japanese  species  not  known  in  Europe, 
22  were  known  from  eastern  but  not  from  western  North 
America.  Exploration  subsequent  to  the  date  of  Gray's 
paper  has  altered  our  knowledge  of  the  distribution  of 
many  species  in  the  region  referred  to,  but  the  broad 
fact  pointed  out  by  Gray  has  only  been  confirmed  by  more 
careful  investigation. 

Several  writers2  have  called  attention  to  the  fact  that 
various  species  of  plants  and  of  invertebrate  animals  are 
confined  to  the  west  of  Ireland  and  North  America. 

1  Gray,  Asa.     On  the  botany  of  Japan,  and  its  relations  to  that  of  North 
America,  etc.     Botanical  Memoirs,  extracted  from  Vol.  VI  (New  Series)  of 
the  Mem.  Amer.  Acad.  Arts  and  Sciences.     Boston  and  Cambridge,  25th 
April,  1859. 

2  E.g.,  Colgan,  N.,  and  R.   W.  Scully.     Cybele  Hibernica.     zd  Ed.  p. 
71.     Scharff,  R.  F.    Proc.  Royal  Irish  Acad.  28: 13.     Nov.,  1909. 


GEOGRAPHICAL  DISTRIBUTION  157 

Among  the  plants  may  be  mentioned  ladies'  tresses  (Spi- 
ranthes  Romanzoffiana)  and  the  seven-angled  pipewort 
(Eriocaulon  septangulare) ;  and  among  animals,  the  land 
snail,  Helix  hortensis. 


FIG.  73. — Asa  Gray  (1810-1888).     Noted  American  botanist  and  student 
of  plant  geography. 

Of  the  various  theories  which  have  been  advanced  to 
explain  the  occurrence  of  identical  species  on  opposite 
shores  of  the  North  Atlantic,  Scharff  enumerates  the 
following  three.1 

i.  Migration  from  Europe  across  Asia  and  a  Bering 
Strait  land-bridge  to  America,  or  vice  versa. 

1  Scharff,  Robert  Francis.  On  the  evidences  of  a  former  land  bridge  be- 
tween northern  Europe  and  North  America.  Proc.  Royal  Irish  Acad. 
288:1-28.  Nov.,  1909. 


158  HEREDITY   AND   EVOLUTION   IN  PLANTS 

2.  Occasional  transport  by  birds  across  the  Atlantic 
Ocean. 

3.  Migration  across  a  direct  Atlantic  land-connection. 
Human  agency  is  generally  rejected,  except  in  cases  where 
it  can  positively  be  demonstrated. 

In  interpreting  the  above  facts  Scharff  argues  that  "the 
interchange  between  the  fauna  and  flora  of  north-western 
Europe  and  north-eastern  America  was  effected  across 
the  northern  land  bridges,"  which  the  facts  of  distribu- 
tion and  other  evidence  indicate  existed  in  pre- Glacial 
times,  and  probably  in  late  Pliocene  and  early  Pleistocene. 

Numerous  alpine  species  have  a  present  discontinuous 
distribution  in  the  lowlands  of  arctic  and  sub-arctic  lati- 
tudes, and  on  lofty  mountain  peaks,  widely  separated, 
in  more  southern  latitudes.  Darwin  called  attention  to 
the  fact  that  "a  list  of  the  genera  of  plants  collected  on  the 
loftier  peaks  of  Java  raised  a  picture  of  a  collection  made 
on  a  hillock  of  Europe;"  and  again  that  "certain  plants 
growing  on  the  more  lofty  mountains  of  the  tropics  in 
all  parts  of  the  world,  and  on  the  temperate  plains  of  the 
north  and  south,  are  the  same  species  or  varieties  of  the 
same  species."  A  striking  illustration  of  this  latter  fact 
is  the  small  white  water-lily  (Castalia  tetragona),  which  is 
found  along  the  Misinaibi  and  Severn  rivers  in  Ontario 
(Canada),  and  at  Granite  Station,  in  northern  Idaho 
(U.  S.  A.),  but  is  not  known  elsewhere  except  in  Siberia 
China,  Japan,  and  the  Himalaya  mountains  (Kashmir). 

The  flora  near  the  summit  of  Mt.  Washington  and  other 
peaks  of  the  White  Mountains,  in  New  Hampshire,  has 
elements  in  common  with  that  of  Labrador.  "In  ap- 
proaching these  mountain  summits,"  says  Flint,1  "one 

1  Flint,  William  F.  The  distribution  of  plants  in  New  Hampshire. 
In  Hitchcock,  C.  H.  The  Geology  of  New  Hampshire,  i:  393.  1874. 


GEOGRAPHICAL   DISTRIBUTION 


159 


is  struck  by  the  appearance  of  the  firs  and  spruces,  which 
gradually  become  more  and  more  dwarfish,  at  length  ris- 
ing but  a  few  feet  from  the  ground,  the  branches  spread- 
ing out  horizontally  many  feet  and  becoming  thickly  inter- 
woven. These  present  a  comparatively  dense  upper 


FIG.  74. — Lapland  rhododendron  (Rhododendron  lapponicum).  Photo- 
graphed on  the  summit  of  Mt.  Madison,  New  Hampshire,  June  25,  1917, 
by  Ralph  E.  Cleland. 

surface,  which  is  often  firm  enough  to  walk  upon.  At 
length  these  disappear  wholly,  and  give  place  to  the  Lap- 
land rhododendron  (Fig.  74),  Labrador  tea,  dwarf  birch, 
and  alpine  willows,  all  of  which,  after  rising  a  few  inches 
above  the  ground,  spread  out  over  the  surface  of  the 


160  HEREDITY  AND   EVOLUTION  IN   PLANTS 

nearest  rock  thereby  gaining  warmth,  which  enables  them 
to  exist  in  spite  of  tempest  and  cold.  These  in  their 
turn  give  place  to  the  Greenland  sandwort,  the  diapensia 
(Fig.  75),  the  cassiope,  and  others,  with  arctic  rushes, 
sedges,  and  lichens,  which  flourish  on  the  very  summit."1 

According  to  Flint,  there  are  about  fifty  strictly  alpine 
species  on  these  summits,  found  nowhere  else  in  New 
England  and  New  York,  except  on  similar  summits,  such 
as  Mt.  Katahdin  in  Maine,  and  Mt.  Marcy  and  Mt. 
Mclntyre  in  New  York  State. 

Incidentally,  it  may  be  remarked  that  a  similar  state- 
ment may  be  made  for  the  animal  life.  Writing  of  the 
insects,  Scudder  says2  that,  "in  ascending  Mt.  Washing- 
ton, we  pass,  as  it  were,  from  New  Hampshire  to  northern 
Labrador;  on  leaving  the  forests  we  first  come  upon  animals 
recalling  those  of  the  northern  shores  of  the  Gulf  of  St. 
Lawrence  and  the  coast  of  Labrador  opposite  Newfound- 
land; and  when  we  have  attained  the  summit,  we  find 
insects  which  represent  the  fauna  of  Atlantic  Labrador 
and  the  southern  extremity  of  Greenland." 

118.  Effects  of  Continental  Glaciation.— The  above 
mentioned  and  other  similar  cases  of  discontinuity  are 
satisfactorily  explained  by  the  advance  and  retreat  of  the 

1  Among  numerous  species  that  have  been  recorded  from  both  Labra- 
dor and  the  peaks  of  the  White  Mountains,  there  may  be  mentioned  the 
following:  Salix  argyrocarpa,  S.  phylicifolia,  S.  herbacea,  S.  uva-ursi, 
Comandra  livida,  Arenaria  groenlandica,  Silene  acaulis,  Oxyria  digyna, 
Cardamine  bellidifolia  laxa,  Saxifraga  rivularis,  Sibbaldia  procumbent, 
Empetrum  nigrum,  Epilobium  Hornemannii,  Loiseleuria  procumbens, 
Rhododendron  Lapponicum,  Phyllodocc  coerulea,  Cassiope  hypnoides, 
Arctostapphylos  alpina,  Vaccinium  ccespiiosum,  Diapensia  Lapponica, 
Veronica  alpina  var.  unalaschcnsis ,  Gnaphalium  supinum. 

3  Scudder,  Samuel  H.  Distribution  of  insects  in  New  Hampshire.  In 
Hitchcock,  C.  H.  The  geology  of  New  Hampshire,  I  :  341.  1874. 


GEOGRAPHICAL   DISTRIBUTION  l6l 

continental  glacier  during  the  Ice  Age.  With  the  advance 
of  the  ice  all  vegetation  was  either  exterminated  or  com- 
pelled to  migrate  southward.  With  the  subsequent 
retreat  of  the  ice  northward  the  glaciated  region  was 
gradually  re-occupied  by  the  encroachment  of  vegetation 
from  the  south,  and  of  this  flora  the  arctic  species  could 
become  permanently  re-established  only  in  what  are  now 
the  arctic  regions,  and  in  the  arctic  or  sub-arctic  climate 
of  the  higher  mountain  tops,  forming  there  what  is  known 
as  a  relict  flora.1  It  has  been  suggested  that,  in  theory, 
alpine  plants  on  high  mountain  peaks  south  of  the  region 
covered  by  the  continental  ice  sheet,  should  not  be  related 
to  arctic  and  sub-arctic  forms.  In  harmony  with  this 
idea  Wallace  has  cited  the  volcanic  Peak  of  Tenerifle 
(Pico  de  Teyde),  in  the  Canary  Islands,  12,000  feet  high, 
where,  above  the  timber  line,  von  Buch  found  only  eleven 
species  of  plants,  eight  of  which  appeared  to  be  endemics ; 
but  all  of  them  were*  related  to  the  plants  of  the  same 
general  region,  growing  at  lower  levels. 

However,  seed-distribution  by  birds  and  winds  and 
other  agencies  has  been  going  on  continually  since  the 
continental  ice  sheet  began  to  recede,  with  the  result 
that  arctic-alpine  and  subarctic-alpine  plants  are  numer- 
ous in  the  alpine  zone  of  higher  peaks  below  the  southern 
limits  of  continental  glaciation.  Thus  the  snowy  cinque- 
foil  (Potentilla  nivea)  is  found,  not  only  throughout  the 
arctic  regions,  but  also  in  the  Alps,  in  alpine  Asia,  and 
in  the  Rocky  Mountains  as  far  south  as  Utah  and  Colo- 

1  The  effect  of  continental  glaciation  on  the  distribution  of  plants  was 
first  noted  by  Edward  Forbes,  but  was  also  worked  out  independently  by 
Darwin  several  years  previous  to  the  publication  of  Forbes's  paper. 
(Darwin,  C.  Life  and  Letters,  i  :  71-72,  372.  New  York,  1901.  See  also 
The  Origin  of  Species,  2:  152.  New  York,  1902.) 


162 


HEREDITY   AND   EVOLUTION  IN   PLANTS 


rado.  According  to  Rydberg1  there  is  evidence  that  it 
has  spread  not  only  in  the  earlier  postglacial  period,  but 
also  in  recent  years.  The  common  arctic  and  sub-arctic 
grass,  Phleum  alpinum,  occurs  as  far  south  as  Arizona  and 


FIG.  75. — Diapensia  lapponica.     Photographed  on  the  summit  of  Mt. 
Madison,  New  Hampshire,  June  25,  1917,  by  Ralph  E.  Cleland. 

California  and  the  Sierra  Madre  of  Mexico,  and  also  in 
Patagonia.  It  is  also  found  throughout  the  Montane 
zone,  from  which  it  might  have  spread  to  the  subalpine, 
following  the  woods  throughout  the  whole  mountain 
system.  Other  similar  cases  might  be  cited. 
1  Letter  from  Dr.  P.  A,  Rydberg  to  the  author. 


GEOGRAPHICAL   DISTRIBUTION  163 

119.  Escapes  from  Cultivation. — Every  case  of  discon- 
tinuous distribution  must  be  carefully  analyzed  by  itself, 
and  care  must  be  taken  not  to  adopt  unwarranted  con- 
clusions. Thus,  certain  cases  of  discontinuity  are  ex- 
plained by  the  escape  from  cultivation  of  forms  introduced 
by  human  agency  for  economic  uses,  and  thus  have  no 
scientific  significance.  The  presence  in  the  Hawaiian 
Islands  of  such  economic  plants  as  sugar-cane,  cocoanut, 
and  others  is  an  apparent  case  of  discontinuity,  but  these 
plants  are  known  to  have  been  introduced  there  by  man, 
and  to  have  escaped  from  cultivation.  Campbell  thinks 
that  the  candle-nut  tree  (Aleurites  moluccana,  the  source 
of  a  commercial  oil)  and  the  mountain  apple  (Eugenia 
malaccensis) ,  which  now  constitute  the  chief  elements  in 
the  lowland  forests  of  Hawaii,  were  also  introduced  by 
man,  and  are  therefore  only  apparent  cases  of  disconti- 
nuity. Among  numerous  illustrations  of  this  in  North 
America  may  be  mentioned  the  paper  mulberry  (Brousone- 
tia  papyrifera),  white  mulberry  (Morus  alba),  hemp, 
(Cannabis  saliva),  stinging  nettle  (Urlica  dioica),  the  day 
lily  (Hemerocallis  fulva) ,  all  natives  of  Europe  and  Asia, 
and  the  tree,  Paulownia,  a  Japanese  species  now  becoming 
established  as  an  escape  from  cultivation  in  New  York, 
New  Jersey,  the  District  of  Columbia,  and  Georgia.  The 
last  two  species  were  introduced  into  North  America  as 
ornamental  plants,  the  hemp  and  white  mulberry,  of 
course,  as  economic  plants,  the  latter  in  connection  with 
the  raising  of  silk  worms. 

Attention  has  recently  been  called  to  the  wide  and  rapid 
spread  of  the  Japanese  honeysuckle  (Lonicera  japonica) 
introduced  in  the  Eastern  United  States  from  Asia. 
Twenty-five  or  thirty  years  ago  this  was  a  comparatively 


1 64  HEREDITY   AND   EVOLUTION  IN   PLANTS 

rare  cultivated  vine,  but  since  that  time,  according  to 
Miss  Andrews,1  ''it  has  spread  over  practically  the  whole  of 
the  Eastern  States,  from  the  Gulf  of  Mexico  to  the  estuary 
of  the  Hudson,  making  itself  equally  at  home  in  the  low 
hammocks  of  the  Coastal  Plain,  on  the  old  red  hills  of  the 
Piedmont  region,  on  the  stony  ramparts  of  the  Lookout 
Plateau,  and  onward  for  a  thousand  miles  up  the  great 
Appalachian  Valley."  The  adaptability  of  the  plant,  as 
indicated  by  this  description  of  its  habitats,  in  no  doubt  a 
large  factor  in  its  rapid  spread,  for  while  it  is  a  profuse 
bloomer  under  cultivation,  it  tends  to  become  weedy,  as 
it  grows  wild,  blossoming  rarely  and  therefore  setting  few 
seeds.  But  its  wide  distribution  must  have  been  accom- 
plished by  the  dissemination  of  its  seeds,  and  in  this  Miss 
Andrews  believes  that  the  most  probable  agents  are  birds, 
to  whose  feet  the  small,  inconspicuous  nutlets,  "embedded 
in  a  mucilaginous  pulp,"  readily  adhere. 

Several  species  (e.g.,  the  fleabane,  Pluchea  fcetida) 
are  found  in  shallow  fresh  water  or  fresh  or  salt  water 
marshes  from  southern  New  Jersey  to  Florida,  and  then 
across  120  miles  of  salt  water  in  Cuba.  In  this  case  it 
seems  clearly  evident  that  the  seeds  have  been  able  to 
undergo  transportation  across  the  Florida  strait  within 
comparatively  recent  times.  Examples  might  be  multi- 
plied, and  in  such  cases  discontinuous  distribution  has 
little  evolutionary  significance  for  the  particular  species 
concerned,  though  the  facts  may  serve  to  throw  light  upon 
other  cases  that  are  significant. 

120.  Endemism.- — On  the  basis  of  the  evolution  theory 
every  species  originated  in  some  one  area  (its  center  of  dis- 

1  Andrews,  E.  F.  The  Japanese  honeysuckle  in  the  Eastern  United 
States.  Torreya,  19  : 37-43.  Mch.  1919. 


GEOGRAPHICAL   DISTRIBUTION  165 

tribution),  where  it  was  at  first  endemic,1  and  whence  it 
gradually  spread  as  far  as  it  could.  This  is  well  illustrated 
in  the  distribution  of  the  Verbenaceae,  one  of  the  higher  and 
therefore  more  recent  families  of  flowering  plants,  com- 
prising about  75  genera  and  1300  species,  occurring 
widely  throughout  tropical  and  temperate  regions.  Of 
104  species  belonging  to  various  genera  in  the  Philippines, 
60  per  cent.,  according  to  Lam,2  are  apparently  endemic. 
These  endemic  forms  have  undoubtedly  been  derived  from 
the  39  non-endemic  species,  and  will,  in  the  course  of  time, 
spread  from  the  Philippines  to  neighboring  islands  and 
thence  to  the  mainland.  About  85  per  cent,  of  the 
flora  of  Hawaii  is  endemic,3  and  even  the  strand  flora, 
while  cosmopolitan  on  the  whole  (the  general  rule  for 
coastal  vegetation),  is  nearly  40  per  cent,  endemic,  a 
surprisingly  high  percentage. 

From  the  facts  of  geographical  and  geological  distri- 
bution, Wallace  deduced  the  following  law:4  Every 
species  has  come  into  existence  coincident  both  in  time  and 
space  with  a  pre-existing  closely  allied  species."  "The  law 
here  enunciated,"  said  Wallace,  "not  merely  explains 
but  necessitates  the  facts  we  see  to  exist,  while  the  vast  and 
long-continued  geological  changes  of  the  earth  readily 
account  for  the  exceptions  and  apparent  discrepancies 
that  here  and  there  occur."  And  again,  "this  law  agrees 

1  Endemic:  found  in  a  given  region,  but  not  elsewhere. 

2  Lam,  H.  J.     The  verbenaceae  of  the  Malayan  Archipelago.     Gronin- 
gen,  1919. 

3  Including,  for  example,  all  the  native  Hawaiian  palms,  belonging  to  the 
genus   Pritchardia.     See     MacCaughey,    Vaughan.     Bull.    Torrey   Bot. 
Club,  45 :  259-277.    July,  1918,  and  Plant  World  21 : 317-328.     Dec.,  1918. 

4  Wallace,  Alfred  Russel.     On  the  law  which  has  regulated  the  introduc- 
tion of  new  species.     Annals  and  Mag.  of  Nat.  Hist.  16,  Ser.2  :  184-196. 
Sept.  1855. 


1 66  HEREDITY   AND   EVOLUTION  IN   PLANTS 

with,  explains  and  illustrates  all  the  facts  connected  with 
the  following  branches  of  the  subject:  ist,  the  system  of 
natural  affinities;  2d,  the  distribution  of  animals  and 
plants  in  space;  3d,  the  same  in  time  .  .  .  4th,  the 
phenomena  of  rudimentary  organs."  And  Wallace  goes 
on  to  show,  in  detail  the  bearing  of  the  law  upon  each  of 
the  four  points  enumerated. 


FIG.  76. — Alfred  Russel  Wallace  (1823-1913).  Co-discoverer,  with 
Darwin,  of  the  principle  of  natural  selection.  Noted  student  of  geo- 
phical  distribution. 

A  quotation  from  Darwin  is  also  pertinent  here:  "It 
is  ...  obvious,"  said  Darwin,  "that  the  individuals  of 
the  same  species,  though  now  inhabiting  distant  and 
isolated  regions,  must  have  proceeded  from  one  spot,  where 
their  parents  were  first  produced  for,  as  has  been  explained, 
it  is  incredible  that  individuals  identically  the  same  should 
have  been  produced  from  parents  specifically  distinct." 

121.  Mutation  and  Discontinuous  Distribution. — Read- 
ing Darwin's  statement  in  the  light  of  the  mutation  theory 


GEOGRAPHICAL  DISTRIBUTION  167 

of  de  Vries,  we  must  of  course  recognize  that,  if  a  mutating 
species  were  widely  distributed,  different  individuals  of  the 
species  in  widely  separated  localities  and  even  with  a  dis- 
continuous distribution,  might  throw  the  same  mutants. 
(Enothera  Lamarckiana,  for  example,  threw  the  same  ele- 
mentary species  (mutants)  in  experimental  pedigree 
cultures  in  Holland  and  in  various  localities  in  the  United 
States.1  Had  0.  Lamarckiana  (contrary  to  fact)  been 
widely  distributed  in  nature,  such  mutants  as  O.  gigas, 
O.  scintillans,  O.  Icevifolia,  and  others  would  possibly  (or 
even  probably)  have  appeared  in  different  and  widely  sepa- 
rated stations,  and  these  elementary  species  might  con- 
ceivably (and  not  improbably)  have  become  established  as 
true  species  of  the  systematist.  When,  therefore,  we 
find  a  given  species  (or  a  larger  group)  in  widely  separated 
localities,  but  not  in  the  intervening  regions,  we  must 
(barring  the  phenomenon  of  mutation  referred  to  above) 
conclude,  either  that  it  has  been  able  to  migrate  across 
barriers  where  it  could  not  become  established  (as  when 
seeds  of  land  plants  are  carried  by  ocean  currents  across 
barriers  of  salt  water),  or  else  it  has  formerly  had  a  con- 
tinuous distribution,  but  has  subsequently  died  out  in 
regions  between  its  present  localities;  in  the  latter  case  it  is 
referred  to  as  a  relict  endemic.  When  these  localities  are 
distant  hundreds  or,  as  is  often  the  case,  thousands  of  miles 
from  each  other,  one  can  readily  understand  that  species 
having  such  discontinuity  of  distribution  must,  other 
things  being  equal,  be  older  than  species  having  continuity 
of  distribution;  they  must  have  existed  long  enough  for  the 
changes  above  mentioned  to  have  taken  place. 

This  principle  is  confirmed  by  the  evidence  of  fossils. 
A  striking  case  is  that  (cited  by  Chodat)  of  Zelkowa, 

'See  pp.  114-117, 


1 68  HEREDITY  AND   EVOLUTION  IN  PLANTS 

related  to  our  modern  elms.  This  genus  comprises 
only  four  living  species,  which  occur  in  only  three  widely 
separated  areas,  namely,  the  far  East  (Eastern  China 
and  Japan),  the  area  between  the  Black  and  the  Caspian 
Seas  (Caucasia),  and  islands  in  the  eastern  Mediterranean 
Sea.  But  a  study  of  the  fossil  evidence  shows  that  during 
a  preceding  geological  age  this  genus  had  a  very  extended 
distribution,  including  central  Europe,  the  Iberian  penin- 
sula, Iceland,  southeast  Greenland,  Labrador,  western 
North  America,  and  Alaska.  Owing  to  profound  changes 
of  climate,  in  the  transition  from  one  geological  age  to 
another,  Zelkowa  was  apparently  unable  to  survive, 
except  in  the  two  restricted  and  widely  separated  areas 
where  it  is  now  found. 

122.  Continuous  Distribution. — Continuous  distribution 
is  of  two  types:  ubiquitous,  like  the  bracken  fern,  and 
isolated,  like  the  redwoods,  Sequoia.  In  the  latter  case 
two  suppositions  are  possible:  either  the  species  or  genus 
is  very  new  and  has  not  had  time  to  spread  (indigenous 
endemic} ;  or  it  is  very  old  and  a  relict  endemic,  as  denned 
above.  Which  of  these  two  alternatives  is  correct  for 
any  given  case  may  be  ascertained  only  on  the  basis 
of  comparative  anatomic  evidence,  or  on  fossil  evidence, 
or  on  both. 

The  motile  sperms  and  the  structure  of  the  wood  of  the 
maiden-hair  tree  (Ginkgo  biloba),  for  example,  point 
without  question  to  affinities  with  an  older  type  of  seed- 
bearing  plants,  the  Cycads.  In  the  case  of  the  genus 
Sequoia,  with  only  two  living  species,  the  coast  redwood 
(S.  sempervirens}  and  the  giant  redwood  (S.  gigantea], 
restricted  in  range  to  one  state,  California,  the  fossil 
evidence  shows  that  these  two  species  are  the  meager 


GEOGRAPHICAL   DISTRIBUTION 


i69 


remains  (relict  endemics)  of  a  genus  of  several  species, 
which,  in  Tertiary  times,  was  widespread  over  most  of 
the  northern  hemisphere  (Fig.  77). 

By  a  like  balancing  of  evidence  we  are  able  to  ascertain 
that  the  ubiquitous  fern  family,  Polypodiacea,  with 
some  200  genera  and  about  3,000  species,  is  a  com- 
paratively modern  group,  while  the  Osmunda  family, 


FIG.  77.— Map  showing  the  known  geographical  distribution  of  Se- 
quoia during  the  Cenozoic  era.  The  cross  indicates  the  only  known  loca- 
tion of  living  specimens.  (After  E.  W.  Berry.) 

with  only  two  (or  possibly  three)  living  genera  and  some 
ten  species,  and  with  wide  but  discontinuous  distribution, 
is  much  older.  The  greater  antiquity  indicated  for  the 
Osmundaceae  by  the  facts  of  their  geographical  distribu- 
tion is  also  attested  by  fossil  evidence,  and  further  by  the 
nature  of  their  spores.  The  spores  when  mature  contain 
chlorophyll,  and  this  fact,  of  itself,  indicates  antiquity; 
for  this  and  other  structural  and  physiological  reasons, 


i  yo 


HEREDITY  AND   EVOLUTION   IN   PLANTS 


they  quickly  perish  unless  they  find  at  once  suitable 
conditions  for  germination  and  development.  Thus  they 
could  not  spread  rapidly  over  large  areas.  In  the  light 
of  these  facts  the  only  logical  inference  is  that  their 
wide  and  discontinuous  distribution  must  have  required 
a  vast  period  of  time.  The  tulip  tree,  represented  now 
by  only  one  genus  (Liriodendron)  and  one  or  possibly 


FIG.  78. — Map  showing  the  known  geographical  distribution  of  the 
bald  cypress  (Taxodium)  in  the  Tertiary  and  Pleistocene.  Tertiary  dis- 
tribution, shaded;  Pleistocene  occurrences  north  of  its  present  limits,  in 
dots;  present  distribution,  black.  (From  Shimer,  after  E.  W.  Berry.) 

two  species,  and  with  discontinuous  distribution  (Eastern 
North  America  and  China),  represents  an  old  type  now, 
perhaps,  on  the  way  to  extinction.  A  similar  statement 
may  be  made  for  Sassafras,  for  the  bald  cypress  (Taxodium, 
Fig.  78),  and  numerous  other  groups. 

In  general  it  may  be  said  that  groups  considered  relatively 
more  primitive  or  ancient  on  morphological  or  paleonto- 


GEOGRAPHICAL  DISTRIBUTION  1 71 

logical  grounds,  are  characterized  by  few  genera  and  a 
restricted  or  (if  wide)  discontinuous  distribution.  Thus 
the  Barberry  family,  one  of  the  relatively  primitive 
groups  of  dicotyledons,  contains  only  about  10  genera 
and  over  130  species,  found  in  temperate  North  America 
and  Asia,  temperate  South  America,  and  sparingly  in  the 
tropics;  the  Nymphaeaceae  (Water  lily  family),  more 
primitive  than  the  Berberidaceae,  contains  only  eight 
genera  and  about  50  species,  of  wide  but  discontinuous 
distribution.  In  contrast  there  may  be  mentioned  the 
gamopetalous  Potato  family  (Solanaceae),  with  about 
70  genera  and  1,600  species,  found  generally  on  every 
continent,  and  in  New  Zealand,  Hawaii,  Australasia, 
and  other  oceanic  and  continental  islands,  and  specially 
abundant  in  the  tropics;  and  also  the  still  more  highly 
developed  Madder  family  (Rubiacea) ,  with  as  many  as  355 
genera  and  5,500  species,  also  of  almost  cosmopolitan 
distribution.  As  a  final  example  among  families  of 
flowering  plants,  there  may  be  mentioned  the  Orchidaceae, 
the  most  highly  developed  of  the  Monocotyledons,  and,  on 
morphological  grounds,  possibly  the  most  recent  family 
of  seed-bearing  plants.  This  family  contains  about  430 
genera  and  over  5,000  species,  of  almost  cosmopolitan 
distribution,  most  abundant  in  the  tropics,  and  gradually 
diminishing  toward  the  poles.  The  seeds  of  orchids  are 
very  tiny,  and  the  embryo  consists  of  a  few  undifferenti- 
ated  cells.  They  are  capable  of  rapid  and  wide 
distribution  (Fig.  780). 

In  the  Nympheaceae  is  the  relatively  primitive  genus, 
Nelumbo,  containing  only  two  species,  one  the  lotus 
(N.  luted},  in  North  America,  the  other  the  Oriental 
lotus  (N.  nucifera),  in  Asia  and  Australasia.  In  the 


172 


HEREDITY  AND    EVOLUTION   IN   PLANTS 


Rubiaceae  is  the  genus  Mitchella,  also  relatively  primitive, 
and  containing  only  two  species,  one  in  Japan,  the  other 
the  Partridge  berry  (M.  repens)  in  North  America. 

123.  Evidence  from  the  Distribution  of  Liverworts.— 
The  geographical  distribution  of  the  lower  cryptogams 
(below  the  ferns  and  their  allies)  has  not  been  the  subject 
of  as  extensive  study  as  that  of  the  ferns  and  flowering 
plants,  but  the  evidence  marshalled  by  Campbell1  in  1907, 
concerning  the  distribution  of  the  liverworts  (Hepaticce), 
illustrates  in  a  striking  manner  the  importance  of  the 


FIG.  780. — Seed  capsule  and  seeds  of  an  orchid. 

facts  of  geographical  distribution  in  endeavoring  to 
determine  the  question  of  the  relative  age  of  a  group  of 
plants.  It  had  been  argued  by  Scott,  in  1906,  that  the 
liverworts  were  probably  of  comparatively  recent  origin 
because  of  the  almost  entire  absence  of  fossil  remains  in  the 
Paleozoic  rocks.  But,  as  Scott  himself  records,  impres- 
sions have  been  described  from  Paleozoic  strata  of  plant 
forms  that  can  be  assigned  only  to  the  Hepaticae,  and 
indeed  to  one  of  the  most  highly  organized  groups — the 

1  Campbell,  Douglas  Houghton.     On  the  distribution  of  the  Heptaticae, 
and  its  significance.     New Phytologist  6  :  203-2 1 2 .     Oct.  1907. 


GEOGRAPHICAL   DISTRIBUTION  173 

Marchantiaceae.  This,  of  course,  means  a  long  period 
of  evolutionary  development  from  similar  forms  to  the 
more  complex,  preceding  the  geological  age  of  the  rocks 
containing  the  fossil  record,  and  one  such  bit  of  positive 
evidence  fully  substantiated,  is  of  itself  sufficient  to 
establish  the  antiquity  of  the  liverworts.  Moreover, 
when  such  testimony  is  in  agreement  with  the  evidence 
derived  from  other  sources,  such  as  comparative  mor- 
phology and  geographical  distribution,  the  fact  of  anti- 
quity would  seem  to  be  reasonably  well  established.  Now, 
in  addition  to  the  evidence  of  comparative  morphology, 
there  are,  as  Campbell  points  out,  certain  facts  of  dis- 
tribution that  can  only  be  satisfactorily  interpreted  on 
the  basis  of  the  comparative  antiquity  of  the  liverworts.1 
The  liverworts  are  a  widely  distributed  group ;  some  of  the 
genera  are  cosmopolitan,  i.e.,  they  are  found  practically 
everywhere,  in  all  continents,  climates,  and  habitats, 
and  widely  on  oceanic  islands.  Riccia  and  Marchaniia 
are  cosmopolitan  genera  of  continuous  distribution. 
Other  genera  are  of  wide,  but  discontinuous  distribution, 
such,  for  example,  as  Targionia,  a  genus  containing  only 
two  species,  which  are  found  in  Southern  and  Western 
Europe,  Africa,  Java,  Australia,  and  Western  America, 
but  are  absent  from  Eastern  America  and  from  most  of 
Asia.  The  familiar  Lunularia  cruciata  of  our  greenhouses 
has  a  distribution  similar  to  Targionia  in  the  eastern 
hemisphere,  but  is  unknown  in  the  western  hemisphere 
except  where  introduced. 

1  Throughout  the  discussion  of  liverworjs  I  have  drawn  freely  on  Camp- 
bell's article,  cited  above,  and  have,  to  a  certain  extent,  adopted  his 
wording,  asking  the  reader  and  the  author  quoted  to  accept  this  statement 
in  lieu  of  frequent  quotes. 


174  HEREDITY   AND    EVOLUTION   IN  PLANTS 

A  third  type  of  distribution  is  that  of  limited  range, 
such  as  has  been  mentioned  above  for  the  venus's  fly- 
trap and  the  giant  redwood  trees.  Among  genera  thus 
distributed  are  Wiesnerella  Javanica  Schiff.,  known  at 
present  only  from  Mt.  Gedeh,  in  Java,  and  Geothallus 
tuberosus  Campbell,  known  only  from  near  San  Diego, 
California.  These  ranges  may  ultimately  be  extended,  as 
was  that  of  Treubia  insignis,  known  for  a  time  only  from 
Mt.  Gedeh,  but  later  found  by  its  original  discoverer 
in  New  Zealand. 

As  already  noted,  in  order  to  become  widely  distributed, 
either  continuously  or  discontinuously,  a  plant  must  either 

1.  Have  reproductive  bodies  capable  of  rapid  distribu- 
tion over  wide  areas,  or 

2.  Possess  sufficient  antiquity  to  have  been  in  process 
of  dissemination  for  a  comparatively  long  period  of  time. 
In  the  former  case,  its  reproductive  bodies  must  be  of  such 
nature  as  to  resist  unfavorable  environment  and  vicissi- 
tudes, during  transit  over  long  distances,  and  be  able  to 
establish  themselves  readily  in  the  new  habitat,  especially 
in  competition  with  the  plants  already  established,  and  pos- 
sibly also  in  an  unfavorable  environment.     Now  the  spores 
of  many  of  the  most  widely  distributed  Hepaticos  are  not 
of  this  nature.     We  can  hardly  explain  the  present  distribu- 
tion of  such  widespread  tropical  genera  as  Dendroceros, 
Monodea,  and  Dumortiera,  says  Campbell,  by  the  theory 
that  their  spores  could  be  carried  across  the  wide  ocean 
barriers  that  separate  the  regions  where  they  now  occur, 
as  the  spores  are  not  of  the  type  that  could  be  carried  long 
distances  without  perishing.     Since  there  are  no  connecting 
forms  in  the  higher  latitudes  that  could  explain  the  passage 
of  these  forms  from  one  tropical  zone  to  the  other,  we  can 


GEOGRAPHICAL   DISTRIBUTION  175 

only  assume  that  these  genera  are  the  little  changed 
descendants  of  ancient,  widely  distributed  types. 

Although  making  a  special  search  for  Liverworts  on 
Krakatoa  in  1906,  Campbell  found  no  specimens,  nor  up 
to  that  time  had  any  other  collector.  Professor  Treub, 
of  the  Botanic  Garden  at  Buitenzorg,  Java,  had  reported 
two  species  of  mosses.  "  Inasmuch  as  Krakatoa  is 
within  sight  of  Java  and  Sumatra,  both  of  which  have 
an  extremely  rich  hepatic  flora,  the  absence  of  these 
plants  from  the  new  flora  of  Krakatoa  is,  to  say  the  least, 
worthy  of  note."  In  a  similar  way  Campbell  argues 
that  the  wide  distribution  of  mosses  (cosmopolitan  in 
the  case  of  the  genus  Sphagnum),  combined  with  the 
inability  of  their  reproductive  bodies  to  withstand  trans- 
portation over  great  distances,  indicates  a  great  antiquity 
for  the  group;  and  this  inference  is  substantiated  by  the 
meager  but  positive  evidence  of  fossil  remains. 

In  a  later  discussion  of  the  origin  of  the  Hawaiian 
flora,  Campbell1  notes  that  the  filmy  ferns,  since  they  are 
hydrophytic  with  a  rain-forest  habit,  and  are,  therefore, 
not  suited  to  transportation  over  wide  stretches  of  ocean, 
must  have  existed  in  Hawaii  since  those  islands  were 
connected  with  some  mainland,  now  submerged.  The 
relatively  shallow  water  between  Hawaii  and  the 
Australasian-Malaysian  regions,  as  compared  to  the 
great  depths  between  Hawaii  and  North  America,  in- 
dicate a  former  mainland  connection  to  the  west,  and 
this  inference  is  further  substantiated  by  the  great  pre- 
ponderance of  Australasian-Malaysian  plants  in  Hawaii 
over  those  represented  in  America.  In  this  connection 

1  Campbell,  T>.  H.  The  origin  of  the  Hawaiian  flora.  Mem.  Torrey 
Bot.  Club,  17:  90-96.  June,  1918. 


176  HEREDITY   AND   EVOLUTION   IN  PLANTS 

is  should  be  noted  that  a  considerable  proportion  of  the 
species  of  the  strand  vegetation  of  Hawaii  are  endemic, 
but  many  of  the  introduced  littorals  are  known  to  be 
transported  by  ocean  currents  from  the  north  Pacific.1 

124.  Distribution  of  Algae.— And  finally,  to  bring  all  the 
great  phyla  under  brief  review,  it  may  be  mentioned  that 
facts  of  distribution  of  the  Algae  point  to  a  great  antiquity 
for  'the  group.     This  is  not  only  in  harmony  with  the 
generally  accepted  evidence  from  comparative  morphology, 
but  is  substantiated  by  fossil  remains,  in  early  Paleozoic 
rocks,  of  calcareous  Siphonogamous  forms  related  to  liv- 
ing calcareous  forms.     The  absence  of  fossil  remains  of 
non-calcareous  green  forms  is  readily  explained  by  the 
delicate  nature  of  their  tissues. 

125.  Hypothesis  of  "Age  and  Area." — As  noted  above 
(p.  165),  an  endemic  species  is  one  found  in  a  given  local- 
ity  but   not  elsewhere.     According  to  some  botanists2 
endemism  is  a  criterion  of  youth.     The  area  occupied 
by  a  species  within  a  given  country,  argues  Willis,  varies 
directly  with  its  age  within  that  country,  that  is,  the  longer 
it  has  been  a  part  of  the  flora,  the  wider  the  area  it  occupies, 
so  long  as  conditions  remain  constant.     But  Willis  enumer- 
ates various  conditions  that  would  interfere  with  the 
operation  of  this  law,  including  "chance"  (i.e.,  causes  not 
understood),  the  action  of  man  (clearing  of  forests,3  etc.), 

1  Twenty-one  littorals  and  eleven  pseudo-littorals,  out  of  a  total  of  over 
75,  are  listed  as  endemic  by  Vaughan  MacCaughey.    Bull.  Torrey  Bot. 
Club,  45:  259-277.    July,  1918. 

2  Willis,  J.  C.     The  relative  age  of  endemic  species  and  other  controver- 
sial points.     Ann.  Bot.  31:189-208.     April,  1917.    James  Small  (see  p. 
148)  has  characterized  Willis's  Age  and  Area  hypothesis,  as  the  most  im- 
portant contribution  to  geographical  botany  since  the  Origin  of  Species. 

3Macrozamia  Moorei  is  being  systematically  exterminated  in  Australia 
because  it  is  poisonous  to  cattle. 


GEOGRAPHICAL    DISTRIBUTION  177 

interposition  of  barriers  (mountains,  broad  deserts,  salt 
water  areas,  sudden  changes  of  climate  from  one  district  to 
the  next,  geological  changes,  natural  selection,  local  adapt- 
ation (the  possession  of  a  character  useful  in  one  country  but 
not  in  another),  the  dying  out  of  occasional  old  species,  the 
arrival  of  a  migrating  species  at  its  climate  limit,  et  cetera. 
But  on  the  whole  the  endemic  species,  says  Willis,  are  the 
youngest.  As  an  illustration  of  the  operation  of  the  hy- 
pothesis of  age  and  area,  Small  (I.e.,  p.  25-30)  mentions 
numerous  Compositae  which  have  limited  distribution, 
although  there  would  seem  to  be  practically  no  limit  to  the 
distance  their  pappose  seeds  can  be  transported  by  wind. 
They  are  limited  (endemic)  because  they  are  young. 

According  to  another  view,1  endemic  species  are  the 
oldest  species  of  a  region;  they  are  either  relicts,  and  thus 
very  ancient,  or  they  represent  types  which  have  been  in 
the  region  so  long  that  their  original  characters  have  been 
lost.  The  latter  are  indigenes,  and  are  spoken  of  as 
indigenous  to  the  country.  Endemics,  according  to 
Sinnott,  contain  a  greater  percentage  of  trees  than  do 
wides  (or  polydemics)-  but,  according  to  the  same  author, 
trees  and  shrubs  are  older  than  herbs,  and  therefore  the 
endemic  woody  species  must  be  older  than  the  herbaceous 
element  of  a  given  flora.  The  hypothesis  of  Willis  demands 
that  herbs  be  considered  as  an  older  form  of  vegetation 
than  trees  and  shrubs,  which,  others  argue,  is  contrary  to 
a  mass  of  evidence.  Trees  are  more  common  as  endemics 
(in  Ceylon,  e.g.,  twice  as  common),  notwithstanding  the 
fact  that  they  spread  less  rapidly  than  herbs.  After  its 

1  Sinnott,    Edmund   W.     The   "age   and   area"  hypothesis  and  the 
problem  of  endemism.     Ann.  Bot.  31:200-216.     April,  1917. 

2  "Wides"  and  "polydemics"  are  used  as  antonyms  of  endemics. 


178  HEREDITY   AND   EVOLUTION   IN   PLANTS 

first  rapid  spread,  says  Sinnott,  a  species  becomes  less 
common  the  older  its  age  of  occupation. 

Reviewing  these  two  theories,  Taylor1  holds  that,  in  the 
flora  of  the  vicinity  of  New  York  at  least,  endemism  is  not 
a  criterion  of  antiquity  nor  of  youth,  for  while  many 
endemics  of  the  flora  of  New  York  and  vicinity  are  very 
recent  (as  the  hypothesis  of  Willis  would  require),  and 
while  some  of  them  are  even  found  in  the  geologically 
recent  portion  of  the  area  (one,  Hibiscus  occuliroseus, 
being  a  salt  marsh  plant  and  therefore  very  'new'), 
other  forms  are  relict  endemics  (p.  167),  and  could  not, 
therefore,  be  of  very  recent  origin.2 

As  an  example  of  relict  (and  therefore  old)  endemics 
(outside  the  local  flora  region  of  New  York,  there  may  be 
cited  the  well  known  case  of  the  giant  and  coast  redwoods 
(Sequoia  gigantea  and  S.  sempervirens} ,  and  the  begonia, 
Hilldebrandia  sandwicensis ,  endemic  in  Hawaii.3 

An  example  of  an  indigenous  (and  therefore  relatively 
recent)  endemic,  is  the  well-known  insectivorous  plant, 
Venus  fly-trap  (Dionaa  muscipula),  a  genus  having  only 
one  species,  i.e.,  monotypic  (Fig.  79) .  This  unique  plant  is 
found  in  sandy  swamps,  only  in  a  narrow  strip  of  country 

1  Taylor,  N.     Endemism  in  the  flora  of  the  vicinity  of  New  York. 
Torreya  16:18-27.     Jan.  1916. 

2  Five  cases  of  apparently  relict  endemism  are  cited  by  Taylor  from  the 
vicinity  of  New  York.     Torreya  16  :  18-27.    Jan-  1916. 

3  The  Begoniaceae  have  scarcely  any  representatives  in  the  islands  of 
the  southern,  equatorial,  and  Northern  Pacific,  but  are  abundant  in  the 
Andes  region  of  South  America  and  Mexico.     The  endemic  begonia  of 
Hawaii  is  regarded  by  MacCaughey  (Bot.  Gaz.  66:273-275.     Sept.  1918) 
as  one  of  several  bits  of  evidence  that  "at  one  time  in  the  history  of  the 
Pacific  basin  the  Hawaiian  islands  were  much  more  closely  associated  with 
the  Andean  and  South  Pacific  regions  than  they  are  at  present.     See  also 
P-  175- 


GEOGRAPHICAL   DISTRIBUTION  179 

about  ten  miles  wide  and  extending  about  40  miles  south  of 
Wilmington,  North  Carolina.  The  yellow  waterlily  (Nym- 
phaa  mexicana  Zuccarini) l  may  also  be  cited  as  an  aquatic 


FIG.  79. — Venus  fly  trap  (Dioncea  muscipula). 

example  of  an  indigenous  endemic,  being  known  only  from 
Florida,  Texas,  and  Mexico.2 

1  Castalia  flava  Greene  (1888). 

2  Conard,  Henry  S.     The  waterlilies,  p.  167  and  213.     Carnegie  Insti- 
tution of  Washington,  Publication  No.  4.     1905. 


l8o  HEREDITY   AND   EVOLUTION   IN   PLANTS 

Again,  as  Taylor  points  out,  most  of  the  recent  endem- 
ics in  the  New  York  flora  are  not  woody,  the  proportion 
of  woody  species  among  the  endemics  (17  per  cent.) 
being  essentially  the  same  as  for  the  entire  flora  (18.2 
per  cent.)  Most  of  the  endemics  are  probably  accounted 
for  by  generic  and  specific  instability,  that  is,  by  the  ten- 
dency of  existing  forms  to  vary,  at  or  near  the  edge  of  their 
range,  and  for  the  variations  to  become  established.  At 
least  one  is  a  case  of  "habitat"  endemism;  that  is,  the 
endemic  species  is  confined  to  a  given  locality  because 
suited  to  the  environment  afforded  by  that  locality.  This 
is  illustrated  by  Prunus  Gravesii,  a  saxitile  form  of  the 
beach-plum  (P.  maritima). 

Many  factors  are  involved  in  the  phenomena  of  en- 
demism, and  here,  as  in  the  case  of  discontinuous  geo- 
graphical distribution,  each  case  must  be  carefully  analyzed 
by  itself.  In  view  of  our  present  restricted  knowl- 
edge, we  can  generalize  only  with  extreme  caution. 

126.  An  Illustrative  Study. — As  an  illustration  of  the 
application  of  evidence  from  various  sources  in  an  en- 
deavor to  decide  the  relative  age  of  two  large  groups  of 
plants,  herbs  and  woody  plants  (trees  and  shrubs),  there 
may  be  mentioned  the  recent  work  of  Sinnott  and  Bailey,1 
who  marshalled  evidence  from  paleobotany,  anatomy, 
phylogeny,  and  phytogeography,  as  bearing  on  the  rela- 
tive antiquity  of  herbaceous  and  woody  plants.  Very 
briefly  summarized ,  their  argument  runs  as  follows: 

i.  A  study  of  fossil  plants  shows  that  the  remains  of 

1  Sinnott,  Edmund  W.  and  Irving  W.  Bailey.  Investigations  on  the 
phylogeny  of  the  Angiosperms:  No.  4.  The  origin  and  dispersal  of 
herbaceous  Angiosperms.  Ann.  Bot.  112:  547-600.  Oct.  1914.  The 
phraseology  of  the  authors  is  freely  incorporated  in  the  above  very  brief 
summary. 


GEOGRAPHICAL   DISTRIBUTION  iSl 

Angiosperms  in  earlier  geological  periods  were  almost  all 
woody.  The  number  of  herbaceous  forms  increases  as 
we  pass  from  older  to  more  recent  strata.  Fossils  of 
herbaceous  plants  are  rarely  found  in  Cretaceous  rocks 
but  become  increasingly  abundant  throughout  the  Ter- 
tiary. Caution  is  necessary  here,  however,  for  the  foliage 
and  other  parts  of  herbs  are  more  tender  and  delicate 
than  those  of  woody  plants,  and  therefore  less  liable  to 
be  preserved  as  fossils.  This  evidence  is  significant  only 
in  connection  with  evidence  derived  from  other  sources. 

2.  A  study  of  the  comparative  anatomy  of  stems  indi- 
cates that  the  continuous  ring  of  wood,  which  character- 
izes the  stems  of  all  trees  and  shrubs,  is  a  more  primitive 
character   than   the  separate  nbro-vascular  bundles  of 
herbaceous  stems.     It  is  suggested  that  a  change  from  a 
woody  to  an  herbaceous  type  may  have  resulted  from 
regional  decrease  in  the  activity  of  the  cambium  layer, 
from  which  the  wood  is  formed  by  cell-division  followed 
by  lignification. 

3.  Evidence    from   phylogeny   shows    that    the    more 
primitive  groups  of  Angiosperms  and  their  probable  an- 
cestors are  composed  overwhelmingly  of  woody  plants. 
In  more  than  half  of  the  families  of  Dicotyledons  there  are 
no  herbaceous  species,  and  the  few  families  which  are 
entirely  herbaceous  are  almost  all  insectivorous  plants, 
water  plants,  parasites,  or  monotypic  families,1  and  hence 
can  lay  no  claim  to  great  antiquity.     Also,  there  is  a 
much  larger  proportion  of  woody  plants   in   the   lower 
groups  of  Angiosperms  (Apetalae  and  Polypetalse)  than 
in  the  higher  groups  (Sympetalas.) 

4.  From   a  study  of  plant  geography  we  learn  that 

1  A  monotypic  family  is  a  family  having  only  one  genus. 


182  HEREDITY   AND   EVOLUTION   IN   PLANTS 

dicotyledonous  herbs  preponderate  in  north  temperate 
regions,  and  woody  plants  in  the  tropics.  The  latter 
climate  probably  approaches  more  nearly  to  that  under 
which  Angiosperms  first  appeared.  Herbs,  having  a 
short  life  cycle  (one  to  two  or  three  seasons)  are  able  to 
survive  periods  of  intense  cold  in  the  form  of  seeds,  and 
would,  therefore,  survive  in  larger  numbers  than  woody 
plants  on  the  advance  and  retreat  of  the  continental  ice 
sheet  of  the  Glacial  period.  This  would  account  for  the 
fact  of  a  much  smaller  proportion  of  woody  plants  in  the 
flora  of  Europe,  for  these  could  not  migrate  southward,  as 
the  ice  encroached,  since  the  mountain  ranges  there  have 
a  general  east-west  trend  (in  contrast  to  the  general  north- 
south  trend  of  American  ranges),  and  southern  migration 
would  necessitate  an  ascent  to  high  altitudes  that  would 
be  fatal  to  temperate  or  subtropical  species. 

The  above  facts  are  not  cited  as  established,  but  only 
to  illustrate  a  method.  There  is  also  evidence  and  argu- 
ment suggesting  the  opposite  conclusion,  namely,  that 
herbaceous  plants  are  older  than  woody. 


CHAPTER  XI 
PALEOBOTANY 

127.  The  Scope  of  Paleobotany.— The  study  of  fossil 
plants,  though  of  course  a  phase  of  botany,  constitutes 
a  science  by  itself,  not  only  covering  a  special  subject 
matter,  but  having  its  own  methods  (technique),  and  pos- 
sessing a  large  literature.     It  is  called  paleobotany.     One 
cannot  pursue  this  study  without  a  knowledge  of  the 
anatomy  and  morphology  of  living  forms.     This  is  neces- 
sary in  order  to  interpret  the  meaning  of  plant  fossils, 
which  often  occur  only  in  small  fragments  of  the  entire 
plant.     Moreover,  one  must  have  a  good  knowledge  of 
at  least  the  elements  of  geology,  since  fossils  are  found  in 
rocks.     One  must  not  only  know  the  geological  age  to 
which  the  fossil-bearing  rock  he  studies  belongs,  but  also 
something  of  the  geological  processes  by  which  fossils, 
and  even  the  rocks  themselves,  are  formed. 

128.  What  is  a  Fossil? — A  fossil  is  any  remains  of  a 
plant  or  animal  that  lived  in  a  geological  age  preceding 
the  present;  these  remains  are  preserved  in  rocks.1     There 
are  two  methods  of  preservation,  namely,  incrustation  and 
petrifaction.     Incrustations    are    merely    impressions    or 

1  By  an  extension  of  the  term  we  also  speak  of  fossil  footprints  of  ani- 
mals, fossil  ripple  marks,  el  cetera.     The  word  fossil  is  derived  from  the 
Latin  jodere  (to  dig),  and  originally  signified  anything  dug  up. 
183 


i84 


HEREDITY   AND    EVOLUTION    IX    PLANTS 


casts  resulting  from  the  encasement  of  the  organ  or 
organism  in  the  rock-forming  material.  The  tissue  itself 
either  decayed  or  became  carbonized,  leaving  only  the 


FIG.  80. — Fossil  incrustations  of  the  foliage  of  two  species  of  Spheno- 
phyllum  from  the  coal  measures  of  Missouri.  (From  U.  S.  Geological 
Survey.) 


impression  of  its  surface  features.  The  well-known 
"fossil  fern-leaves,"  found  in  coal  mines, are  of  this  nature. 
The  tissues  of  the  plant  were  transformed  into  coal, 


PALEOBOTANY  185 

leaving  the  impression  or  cast  on  the  adjacent  shale.  The 
first  stage  in  this  process  may  often  be  observed  in  the 
autumn,  when  impressions  .of  recently  fallen  leaves  are 
made  on  the  surface  of  wet  mud.  Obviously  from 
such  fossils  we  can  learn  nothing  of  internal  structure 
(Fig.  80). 

Petrifactions  are  formed  by  the  gradual  replacement 
of  the  organic  tissue  by  mineral  matter,  usually  carbonate 
of  lime  (CaCOs)  or  silicic  acid  (HUSiO,!).  In  this  process 
the  tissues  become  soaked  with  a  saturated  solution  of 
the  given  mineral,  which  is  gradually  deposited  from  solu- 
tion, and  takes  the  place  of  the  original  organic  matter. 
By  this  means  the  most  minute  details  of  microscopic 
structure  are  preserved,  even  in  some  cases  the  nuclei 
and  other  cell-contents  (Figs.  97  and  100). 

129.  Conditions  of  Fossil-formation.— In  order  to 
understand  how  fossils  come  to  be  formed,  we  must  pic- 
ture to  ourselves  certain  geological  processes  now  in 
operation — the  initial  stages  of  rock-formation.  Rocks 
are  of  two  kinds,  igneous  and  sedimentary.  Igneous  rocks 
result  from  the  cooling  of  molten  lava  poured  out  on 
the  surface  or  injected  into  crevices  by  volcanic  action. 
Such  rocks  never  contain  fossils,  as  the  intense  heat 
necessary  to  melt  the  rock  destroys  all  trace  of  organic 
matter. 

Sedimentary  rocks  are -formed  by  the  deposit  under 
water  of  the  sediment  formed  by  weathering  and  erosion 
and  transported  by  streams.  This  deposit  may  occur 
along  the  flood-plains  or  at  the  mouths  of  streams  empty- 
ing into  inland  lakes  or  into  the  ocean.  In  addition  to 
rock-sediment  eroded  from  the  surface  of  the  land,  streams 
also  transport  quantities  of  plant  (and  animal)  frag- 


l86  HEREDITY   AND   EVOLUTION    IN   PLANTS 


PALEOBOTANY 


l87 


ments,  leaves,  stems,  pieces  of  bark,  fruit,  flowers,  pollen 
and  spores,  roots,  and  even  entire  plants.     These  natur- 


FIG.  82. — Diagram  illustrating  the  gradual  filling  up  of  lakes  by  the 
encroachment  of  vegetation,  and  also  the  stages  in  the  origin  of  peat  and 
marl  deposits  in  lakes.  The  several  plant  associations  of  the  Bog  series, 
displacing  one  another,  belong  to  the  following  major  groups:  (i)  O.  W., 
open  water  succession;  (2)  M.,  marginal  succession;  (3)  S.,  shore  succes- 
sion; (4)  B.,  bog  succession,  comprising  the  bog-meadow  (Bm),  bog-shrub 
(Bs)  and  bog-forest  (Bf);  and  (5)  M.  P.,  mesophytic  forest  succession 
(C/.  Fig.  81.)  (After  Bray.) 

ally  become  buried  in  the  mud  and  sediment  wherever 
deposition  takes  place,  and  when  the  deposit  becomes 


1 88  HEREDITY   AND   EVOLUTION   IN   PLANTS 

converted  into  rock  the  organic  remains  may  become  con- 
verted into  fossils  by  either  of  the  processes  described 
above.  Swampy  regions  are  especially  favorable  to  the 
preservation  of  plant  and  animal  remains  as  fossils,  as  is 
illustrated  in  Figs.  81  and  82. 

130.  Metamorphism.— After    sedimentary    rocks    are 
once  formed  they  are  subject  to  various  changes.     The 
amorphous  carbonate  of  lime,  of  limestone  rocks,  may  be 
transformed  into  crystals  of  calcite  until  marble  results; 
thin  flakes  of  mica  may  form  in  clay  rock  in  thin  sheets, 
transforming  the  rock  into  slate;  vegetable  deposits  in 
the  form  of  peat  may  become  transformed  into  anthracite 
coal  and  graphite;  molten  lava  poured  out  on  the  surface 
or  into  crevices  of  sedimentary  rocks  may  fuse  the  adja- 
cent material,  causing  contact  metamorphism;  while  the 
heat  engendered  over  larger  areas  by  mountain  folding, 
or  by  the  weight  of  superincumbent  strata1  may  cause 
regional  metamorphism.     Obviously  such  changes,  espe- 
cially those  caused  by  heat,  result  in  the  complete  de- 
struction of  all  plant  or  animal  remains  or  impressions, 
and  thus  fossil  records  over  large  areas,  and  representing 
vast  periods  of  geologic  time,  have  been  obliterated. 

131.  Stratification  of  Rocks.— Changes  in  the  relative 
level  of  sea  and  land  have  occurred  many  times  in  the 
geological  past,  so  that  submerged  areas  of  sedimentation 
in  one  period  have  become  areas  of  dry  land,  undergoing 
erosion  in  another;  and  vice  versa,  areas  of  erosion  have 
become   areas   of   sedimentation.     As   a   result   of   this, 
rocks  occur  in  layers,2  the  deeper  lying  layers  (with  ex- 

1  Some  rocks  are  buried  under  more  than  40,000  feet  of  strata,  and  the 
temperature  increases  approximately  i°F.  for  every  50  to  60  feet  of  depth. 

2  Several  layers  form  a  stratum,  or  bed. 


PALEOBOTANY  I 89 

ceptions  readily  explained  by  geologists)  being  older  than 
those  above,  or  nearer  the  surface.  Moreover,  as  a  result 
of  a  second  submersion  following  elevation  and  erosion, 
subsequent  layers  were  often  deposited  with  an  uncon- 
ormity  on  the  weathered  and  eroded  surface  under- 
neath. 

By  the  presence  of  fossil  imprints  of  rain  drops,  foot- 
prints, ripple  marks,  and  mud  cracks,  and  by  the  character 
of  the  plant  and  animal  fossils  which  they  contain,  we 
know  that  most  sedimentary  rocks  were  deposited  in 
shallow  water,  not  far  from  the  shore  line.  But  since 
these  same  rocks  may  have  a  thickness  of  thousands  of 
feet  we  know  the  area  of  sedimentation  must  have  been 
slowly  sinking  while  the  sediment  was  being  deposited. 
As  a  result  of  the  enormous  pressure  of  the  overlying 
material,  of  the  deposit  of  cementing  substances  from 
solution,  and  of  other  causes,  the  sedimentary  deposits 
became,  in  time,  converted  into  solid  rock. 

132.  Classification  of  Rock  Strata.— By  a  study  of  the 
fossils  which  the  rocks  contain,  geologists  have  been  able 
to  classify  the  various  strata  according  to  their  age. 
As  a  result  of  the  period  of  erosion,  indicated  by  un- 
conformity, the  transition  from  the  stratum  of  one  age 
to  that  of  another  is  often  abrupt,  the  fossils  in  successive 
periods  being  quite  characteristic  of  the  given  stratum 
or  period.  In  other  cases,  as  for  example  between  the 
Silurian  and  Devonian  in  New  York  State,  there  is  no 
unconformity,  and  this  renders  it  more  difficult  to  decide 
just  where  the  plane  of  division  lies.  The  names  and 
order  of  occurrence  of  the  known  rock  strata  are  given  in 
the  following  table,  the  older  rocks  being  at  the  bottom, 
the  most  recently  formed  at  the  top. 


i  go 


IIKRKDITY    AND    EVOLUTION    IN    PLANTS 


TABLE  II. — TABLE  OF  GEOLOGICAL  TIME 


Era 


Cenozoic 


Mesozoic 


Paleozoic 


Archean 


Quaternary 


Tertiary 


Secondary 


Primary 


Period 

Holocene 

(recent,  or  the  present) 
Pleistocene 

(ice  age) 

Pliocene 
Miocene 
Oligocene 
Eocene 

Upper  Cretaceous 
Lower  Cretaceous 
(Comanchean) 
Jurassic 
Triassic 

Permian 

Upper  Carboniferous 

(Pennsylvanian) 
Lower  Carboniferous 

(Mississippian) 
Devonian 
Silurian 
Ordovician 
Cambrian 

Huronian 
Laurentian 


133.  Paleogeography. — By  changes  in  the  relative  level 
of  the  land  and  sea,  above  referred  to,  rocks  contain- 
ing fossils  may  be  elevated  as  dry  land,  and  frequently 
as  mountains,  so  that  remains  of  marine  organisms,  as 
well  as  of  others,  are  often  found  at  high  elevations.  In 
some  cases  forests  near  the  seashore  have  been  submerged, 
and  covered  over  with  sediment,  then  elevated  again  as 
dry  land,  so  that  subsequent  excavations  have  revealed 
the  fossilized  trunks  and  stumps  (Figs.  83  and  84).  Thus 


PALEOBOTANY 


IQI 


FIG.  83. — Fossil  tree  stumps  in  a  carboniferous  forest,  Victoria  Park, 
Glasgow.     (Cf.  Fig.  84.)     (After  Seward.) 


Fie.   84. — Part  of  a  submerged  forest  as  seen  at  low  tide  on  the  Cheshire 
coast  of  England.     (Cf.  Fig.  83.)     (After  Seward.) 


192  HEREDITY   AND   EVOLUTION    IN   PLANTS 

it  is  seen  that,  by  a  study  of  fossils,  we  may  not  only  learn 
of  their  structure  and  thus  fill  in  many  of  the  gaps  in 
the  evolutionary  sequence  left  by  a  study  of  forms  now 
living,  but  we  may  also  learn  of  the  distribution  of  plants 
and  animals  in  previous  geological  ages — in  other  words, 
we  have  the  basis  for  a  science  of  fossil  geography  or 
paleo  geography. 

134.  Plant  Migrations.  — With   the   development   of 
Paleogeography,  a  clearer  conception  of  the  location  and 
changes  of  the  continental  areas  of  the  past  is  gradually 
being  gained.     As  a  consequence,  plant  geography  is  a  sub- 
ject of  increasing  interest  to  the  paleobotanist.     More- 
over, geology,  the  fossil  record,  and  the  present  zonal 
grouping  of  plants  indicate  that,  in  the  past,  the  polar 
areas,  then  much  warmer  than  now,  must  have  been  fruit- 
ful in  new  species.1     High  mountains  or  plateaus  are  also 
suggested  as  homes  of  plastic  races.2     In  the  tropics  en- 
vironments are  more  nearly  static,  and,  it  is  reasonable 
to  suppose,  less  likely  to  favor  variation.     It  is  knownthat 
once  established,  many  species  move  most  readily  along 
the  geologic  formation  which  supplies  the  exact  soil  con- 
stituents  most  favorable   to    their  growth,  the  rate  of 
movement  often  being  rapid.     Flotation  of  seeds  is  also 
a  factor.     The  facts  here  briefly  cited  rest  on  the  obser- 
vations of  a  large  number  of  investigators,  extending  over 
more  than  a  century. 

135.  Distribution  of  Plants  in  Time. — In  addition  to 
the  distribution  of  plants  in  space  (plant  geography),  the 
problem  of  theirj^distribution  in  geologic  time  is  one  of 

1  Owing  to  the  precession  of  the  equinoxes  these  areas  undergo  an  ex- 
treme variation  in  the  length  of  winter  and  summer  of  37  days  every,  12,934 
years. 

2C/.  pp.  148-149. 


PALEOBOTANY 


193 


absorbing  interest  and  importance.  The  following  table 
indicates  the  known  distribution  of  the  various  plant 
groups  from  the  earliest  geologic  time  to  the  present. 

TABLE  III. — DISTRIBUTION  OF  PLANTS  IN  GEOLOGIC  TiME1 


Division 

Subdivision,  class, 
or  order 

Range 

Common 
name  or 
example 

f  Spermato- 

6 
g 
£     Monoootyled  wes 
o      Dicotyledones 

I 

Comanchean  to  present 
Comanchean  to  present 

Oaks 
Grasses 

(  Cycadophyta 

§     Gnetales 
H     Coniferales 
&    Ginkgoales 
c  ]  Cordaitales 
S     Cycadales 
O  [  Cycadofilicales 

(Fossil  record  scant) 
Permian  to  present 
Permian  to  present 
Devonian  to  Jurassic 
Permian  to  present 
Devonian  toComanchian 

Ephedra 
Pines 
Ginkgo 
Cordaites 
Cycads 
Neuropteris 

f  Lepidophyta 
I  Calamophyta 

"I 

I  Pteridophyta 

Lycopodiales 

Equisitales 
Sphenophyllales 

Filicales 

Devonian  to  present 
Devonian  to  present 
Devonian  to  Permian 

Devonian  to  present 

Club  mosses 
Horsetails 
S  p  h  e  n  o  - 
phyllum 
Ferns 

II.  Bryophyta 

Musci 

Hepaticx 

Tertiary  to  present 
Tertiary*  to  present 

Mosses 
Liverworts 

I.  Thallophyta 

Fungi 
Alga 
Diatomeae 
Schizophyta 
Myxomycetse 

Silurian  to  present 
Pre-Cambrian  to  present 
Jurassic  to  present 
Pennsylvanian  to  present 
(Fossil  record  lacking) 

Fungi 
Seaweeds 
Diatoms 
Bacteria 
Slime-molds 

'Modified  from  Shimer.         2See,  however,  p.  172. 

136.  Gaps  in  the  Fossil  Record. — In  the  Origin  of 
Species  Darwin  called  attention  to  the  paltry  display  of 
fossils  in  our  museums,  as  evidence  of  how  little  we  really 
know  of  the  plant  and  animal  life  of  past  ages.  "The 
number,  both  of  specimens  and  of  species,  preserved  in 


IQ4  HEREDITY   AND   EVOLUTION   IN   PLANTS 

our  museums,"  says  Darwin,  "is  absolutely  as  nothing 
compared  with  the  number  of  generations  which  must 
have  passed  away  during  a  single  formation."  The 
meagerness  of  the  record  is,  of  course,  due  in  part  to  the 
relatively  small  area  explored  in  proportion  to  the  whole ; 
but  there  are  other  reasons  much  more  serious,  because 
they  represent  opportunities  lost  forever.  Among  them 
are  metamorphosis,  explained  above,  and  the  fact  that 
many  of  the  organisms  of  the  past  were  composed  wholly 
or  largely  of  soft  tissues,  which  were  entirely  destroyed,  by 
decay  or  otherwise,  in  the  process  of  rock-formation. 
Such  plants,  for  example,  as  Spirogyra  and  many  other 
algae,  the  fleshy  fungi,  and,  among  animals,  jelly-fish, 
earthworms,  and  others,  would  form  fossils  only  under 
exceptionally  favorable  circumstances,  if  at  all. 

But  there  is  an  even  more  effective  cause  of  oblitera- 
tion of  the  fossil  record  in  the  long-continued  erosion  and 
denudation  represented  by  unconformity  in  the  rock 
strata.  In  many  cases  only  a  small  proportion  now  re- 
mains of  the  thickness  of  a  rock  stratum  originally  de- 
posited, and  all  traces  of  the  plant  and  animal  life  that 
may  have  existed  on  the  denuded  area  have  thus  been  ob- 
literated forever.  These  blank  intervals  between  suc- 
cessive periods  were  of  vast  duration. 

"I  look  at  the  geological  record,"  said  Darwin,  "as  a 
history  of  the  world  imperfectly  kept,  and  written  in  a 
changing  dialect;  of  this  history  we  possess  the  last 
volume  alone,  relating  only  to  two  or  three  countries. 
Of  this  volume,  only  here  and  there  a  short  chapter  has 
been  preserved;  and  of  each  page,  only  here  and  there  a 
few  lines.  Each  word  of  the  slowly  changing  language, 
more  or  less  different  in  the  successive  chapters,  may 


PALEOBOTANY  IQ5 

represent  the  forms  of  life,  which  are  entombed  in  our 
consecutive  formations,  and  which  falsely  appear  to  have 
been  abruptly  introduced."1  These  views  have  received 
added  emphasis  from  the  recent  development  of  Paleo- 
geography. 

137.  Factors  of  Extinction. — The  question  may  natu- 
rally arise,  "Why  did  the  species  common  in  previous  geo- 
logical ages  die  out,  giving  place  to  newer  forms?"  The 
answer  is  found  in  the  facts  of  struggle  for  existence  and 
survival  of  the  fittest.  In  the  words  of  the  great  American 
botanist,  Asa  Gray,  species  may  continue  only  "while 
the  external  conditions  of  their  being  or  well-being  con- 
tinue." The  struggle  may  be  with  other  organisms  or 
with  the  physical  conditions  of  the  environment.  Among 
the  more  important  factors  of  extinction,  may  be  men- 
tioned the  following: 

1.  Struggle  with  Other  Plants  for  Adequate  Space. — This 
is  illustrated  in  a  simple  way  by  the  crowding  out  of  culti- 
vated plants  by  weeds  in  a  neglected  garden,  or  of  grass  by 
dandelions  or  chickweed  in  a  lawn.     By  more  rapid  ger- 
mination and  growth,  and  by  other  "weedy"  character- 
istics, the  weeds  get  the  start  of  the  cultivated  plants, 
occupying  all  available  space,  and  choking  them  out. 

2.  Attacks   of  disease-causing  parasites,   e.g..  chestnut 
trees  by  a  parasitic  fungus,  elm  tress  by  the  elm  tree  beetle. 

3 .  Changes  of  Environment  too  Great  or  too  Rapid  to  Per- 
mit of  Readjustment. — Plants  are  plastic  organisms,   and 
can  adapt  or  readjust  themselves  to  considerable  environ- 
mental change,  but  there  are  limits  of  speed  and  amount 
of  change  beyond  which  readjustment  is  not  possible,  and 
the  plant  must  consequently  perish.     If  such    changes 

1  Darwin,  C.     "Origin  of  Species,"  vol.  2,  p.  88.  New  York,  1902. 


196  HEREDITY   AND    EVOLUTION  IN   PLANTS 

involve  the  entire  area  of  distribution  of  the  species  con- 
cerned, the  species  will,  obviously,  become  extinct.  The 
following  nine  factors  (paragraphs  4-12)  are  specific 
instances  of  this. 

4.  Diminished  Water  Supply. — Aquatic  plants  may  be 
destroyed  by  the  draining  of  a  pond  or  lake;  hydrophytic 
forms  'by  the  drying  up  of  a  swamp.     Sometimes  forms 
suited  to  conditions  of  moderate  water  supply  (hydro- 
phytes) are  destroyd  by  the  conversion  of  wide  areas  into 
desert  regions,  as  has  doubtless  occurred.     If  such  changes 
are  gradual,  resting  spores  (e.g.,  Spirogyra],  winter  buds 
(e.g.,  Utricularia,  Elodea,  Vallisneria),  and  seeds  readily 
transported  by  wind  (e.g.,  cat- tail)  enable  the  species  to 
become  reestablished  in  a  new  location,  but  not  so  when 
the  changes  are  too  abrupt,  or  cover  too  wide  an  area. 

5.  Temperature  changes,  when  too  abrupt,  too  extreme, 
or  too  long  continued.     When  the  continental  ice-sheet 
advanced    southward    during    the    glacial  period,  many 
forms,  adapted  only  to  temperate  conditions,  became  ex- 
tinct.    Fossils    of   extinct    tropical   plants    are  found  in 
Greenland,  which  is  now  undergoing  a  glacial  period. 

6.  Volcanic  eruptions,  such,  for  example,  as  those  of 
Mount  Pelee,  which  occurred  in  1902,  on  the  island  of 
Martinique,  W.  I.,  often  destroy  all  signs  of  life  over  a 
radius   of   many   miles.     In   the  states   of  Washington, 
Oregon,  and  Idaho  floods  of  molten  lava,  covering  thou- 
sands of  square  miles,  have,  during  a  previous  geological 
age,  been  poured  out  over  the  surface,  forming  a  wide 
plateau. 

A  great  volcanic  eruption  in  Alaska,  in  prehistoric 
times,  covered  an  area  of  over  140  square  miles  with  a 
deposit  of  ash  and  pumice  varying  in  thickness  from  a 


PALEOBOTANY  197 

few  inches  near  the  margin  to  some  300  feet  near  the  crater. 
In  1883  the  eruption  of  Krakatoa,  in  the  Straits  of  Sunda, 
killed  practically  all  the  plants  and  animals  on  an  island 
of  five  square  miles  in  area,  and  on  neighboring  islands ; 
a  part  of  the  island  was  completely  blown  away,  leaving 
only  deep  water.  So  recently  as  1912  the  eruption  of 
Katmai,  in  Alaska,  spread  a  layer  of  ash  nearly  a  foot  deep 
over  the  entire  surface  of  Kodiak  Island,  one  hundred 
miles  from  the  volcano,  and  killed  all  the  herbaceous  vege- 
tation, leaving  only  trees  and  bushes.  It  is  almost  certain 
that  many  species  of  plants  and  animals  have  become  ex- 
tinct by  such  agencies.  Not  only  the  lava,  but  poisonous 
gases  that  fiill  the  air  during  volcanic  eruptions,  may 
prove  fatal  to  plant  and  animal  life. 

7.  Encroachment    of    salt    water    in    coastal    regions, 
caused  by  changes  in  the  level  of  the  land,  resulting  in  the 
killing  of  fresh- water  vegetation.     According  to  Fernald, 
one  of  the  sundews,  Drosera  filiformis,  is  known  to  occur 
in  only  two  regions,  namely  along  the  Gulf  coast  from  Flor- 
ida to  Mississippi,  and  along  the  Atlantic  coast  from  Mary- 
land to  Massachusetts  (Fig.  85).     Its  extinction  in  the 
intervening  region  is  explained  by  the  subsidence  and 
drowning  of  a  former  high  continental  shelf,  along  which 
this   and   other  species  migrated  northward  during  the 
late  Tertiary.     If  a  similar  subsidence  should  occur  in  the 
two   limited  regions  where   the  species  is  now  found  it 
would  become  extinct  unless,   by  some  combination  of 
circumstances,  it  could  migrate  and  become  established 
in    new    localities.     It    is  not  unlikely  that  species  have 
often  been  exterminated  in  this  way. 

8.  Encroachment   of  Fresh   Water  over  Land  Areas. — 
Previous  to  about  the  year  1900,  the  Sal  ton  basin,  in 


198  HEREDITY   AND   EVOLUTION   IN   PLANTS 

lower  California,  was  a  saline  area  of  a  so  pronounced 
desert  type  that  its  flora  contained  less  than  140  species 
of  ferns  and  flowerng  plants,  five  of  which  were  endemic. 
During  the  winter  of  1904-1905  the  fresh  waters  of  the 
Colorado  River  began  to  debouche  into  this  basin,  and 
by  early  1907  had  formed  a  brackish  lake,  over  80  feet  deep 
and  of  about  450  square  miles  in  area,  known  as  the  Salton 
Sea.  At  the  end  of  ten  years  it  still  had  an  area  of  some- 
what less  than  300  square  miles.  Some  three  or  four 
hundred  years  previously  the  entire  Salton  Basin  was 


FIG.  85. — Sketch  map  showing  the  geographical  distribution  of  the  sun- 
dew, Droscra  Jiliformis.     (After  M.  L.  Fernald.) 

occupied  with  a  lake  of  over  2,000  square  miles  in  area, 
which,  in  turn,  had  dried  up  and  given  place  to  the  desert 
conditions  above  mentioned.  It  is  not  improbable  that 
such  drastic  changes  as  this  may  have  resulted  in  the 
obliteration  of  one  or  more  species,  though  the  flora  was 
not  well  enough  known  previous  to  the  last  inundation 
to  make  a  definite  statement  on  this  point  possible.  For 
example,  the  presence  there  of  endemic  species  was  not 
known  until  the  recent  botanical  survey  of  the  region 
lying  between  the  late  water  level  and  that  of  the  ancient 


PALEOBOTANY  I 99 

sea.  According  to  MacDougal,1  if  the  water  had  risen  in 
1907  to  its  ancient  level  of  three  or  four  hundred  years 
ago,  it  would  have  destroyed  all  these  endemic  species. 

7 .  Transformation  of  fresh  water  lakes  into  salt  lakes, 
as.in  the  case  of  the  Caspian  Sea,  and  the  Great  Salt  Lake 
of  Utah  (i  8  per  cent.  salt).  This  change  gradually  extermi- 
nates plant  and  animal  life  until  the  given  body  of  water 
becomes  a  true  "dead"  sea,  where  practically  nothing 
remains  alive,  as  in  the  Dead  Sea  (24  per  cent.  salt). 
A  more  extreme  case  yet  is  Lake  Van,  in  Turkey,  where 
saline  matter  constitutes  over  one-third  of  the  contents. 
In  the  last  stages  of  such  transformations  the  lake  may  give 
place  to  a  salt  marsh  or  plain  (salina).  South  of  Lake 
Titicaca,  in  the  Andes  Mts.  of  Bolivia,  ar  several  salinas, 
one  of  some  4000  square  miles  in  area,  with  a  layer  of 
salt  three  or  four  feet  thick. 

10.  Disturbance  of  Symbiotic  Relationships. — The  inter- 
relationships of  organisms  are  very  complex,   affording 
innumerable  opportunities  for  extinction  by  a  disturbance 
of   adjustments.     Shade-loving   forms   in   a   forest   may 
perish  by  the  destruction  of  those  affording  the  shade; 
obligate  parasites  may  perish  from  the  destruction  of  the 
necessary  host;  plants  dependent  upon  certain  insects  for 
cross-pollination  may  perish  on  account  of  the  extinction 
of  the  necessary  insects. 

11.  Diminution  of  Carbon  Dioxide  in  the  Atmosphere. — 
There  are  reasons  for  thinking  that  in  certain  past  ages 
the  atmosphere  was  richer  than  now  in  carbon  dioxide, 
and  that  that  condition  was  favorable  to  the  development 
of  certain  vegetatively  vigorous  species  which  cannot  live 
in  an  atmosphere  like  the  present,  having  a  smaller  per- 
centage of  carbon. 

1  In  a  letter  to  the  author. 


2CO  HEREDITY   AND   EVOLUTION  IN   PLANTS 

12.  Denudation  of  the  Land  Surface. — In  the  course  of 
ages  even  lofty  mountains  are  planed  down  by  erosion, 
and  the  arctic  and  sub-arctic  species  of  the  high  altitudes 
thus  undergo  extinction.  Furthermore,  erosion  may  be 
coupled  with  general  subsidence.  In  fact,  not  only  do 
geologists  now  recognize  numerous  old  mountain  "roots," 
such  for  example  as  the  Adirondack  region  of  New  York 
State,  but  there  are  also  abundant  evidences  of  periodic 
emergencies  and  subsidence  of  areas  of  continental  extent, 
quite  throughout  geologic  time.  The  climatic  and  other 
environmental  disturbances  accompanying  such  changes 
would  inevitably  result  in  the  extinction  of  certain  species. 
(See  also  1  129.) 


CHAPTER  XII 
THE  EVOLUTION  OF  PLANTS  (Concluded) 

138.  Evidences  from  Fossil  Plants.— The  study  of  fossil 
plant   remains    has  greatly  enlarged  our  knowledge   of 
the  course  of  plant  evolution,  filling  in  gaps  derived  from 
the  study  of  living  forms,  and  affording  new  facts,  not 
disclosed  by  the  study  of  plants  now  living.     Like  the 
study  of  comparative  anatomy  and  life  histories,  paleo- 
botany  teaches  us  that  there  has  been  a  gradual  evolu- 
tionary progress  from  the  simple  to  the  more  complex,  but 
it  has  also  disclosed  the  fact  that  some  of  the  complex 
forms  are  much  more  ancient  than  had  been  inferred  from 
the  study  of  living  plants  only. 

139.  Discovery  of  Seed-bearing  Ferns. — For  example, 
remains  of  seed-bearing  plants,  quite  as  highly  organized 
as  those  of  to-day,  are  found  far  back  in  the  earliest  fossil- 
bearing  strata  of  the  Paleozoic.     Great  forest  types  _ex- 
isted  as  early  as  the  Devonian.     Later  in  the  Carboniferous 
occur  many  seed-bearing  ferns.     These  have  been  called 
Cycadofilicales  (cycadaceous  ferns),  or,  by  some,  Pterido- 
sperms.     Recent   studies    have   disclosed    the   fact    that 
most  of  the  fossil  plants  from  the  Carboniferous  coal- 
bearing  strata,  formerly  thought  to  be  ferns,  are  not  even 
cryptogams,  but  are  these  fern-like  seed-bearing  plants. 
The  best  known  pteridosperm  is  Lyginodendron  oldhamium 
(Fig.  86),  first  described  from  fossil  leaves,  in  1829,  as 
a  tree-fern,  under  the  name  Sphenopteris  Hoeninghausi. 
After  investigations  extending  over  nearly  90  years, "  we  are 


HEREDITY    AND    EVOLUTION   IN    PLANTS 


now  in  position  to  draw  a  fairly  complete  picture  of  the 
plant  as  it  must  have  appeared  when  living. 

"It  was  in  effect  a  little  tree-fern,  with  long,  slender, 
sometimes  branched,  stem,  4  centimeters  or  less  in  diame- 


FIG.  86,—Lyginodcndron  oldkamiitm.  Pinna  of  a  microsporophyll, 
found  in  an  ironstone  nodule.  Before  its  identity  was  established  this 
specimen  was  named  Crossolhcca  Hoeninghausi.  The  somewhat  peltate 
fertile  pinules  on  the  ultimate  branches,  bear  each  a  fringe  of  micro- 
sporangia  about  3  mm.  long.  The  appearance  has  been  likened  to  that 
of  a  fringed  epaulet.  (After  Scott,  from  a  photo  by  Kidston.) 

ter,  and  provided  with  spines  by  means  of  which  it  prob- 
ably climbed  on  its  neighbors.  The  foliage  was  disposed 
spirally  and  consisted  of  relatively  very  large,  finely 
divided  fronds  with  small,  thick  pinnules  with  revolute 


THE    EVOLUTION    OF   PLANTS 


203 


margins,  suggesting  a  xerophytic  or  halophytic  habitat. 
The  stem  in  the  lower  portion  gave  rise  to  numbers  of 
slender  roots,  some  of  which  appear  to  have  been  aerial 
in  their  origin.  These  grew  downward  and  often  branched 
where  they  entered  the  soil. 


FIG.  87. — Young  leaf  of  the  Cycad,  Bowenia  serrulala.  Comparison 
of  this  with  a  leaf  of  the  fern  Angiopteris  (Fig.  88)  shows  how  difficult 
it  might  be  to  decide  from  a  fossil  leaf  whether  the  plant  was  a  cycad  or  a 
fern.  (Cf.,  also,  Fig.  91.)  (Photo  from  specimen  in  Brooklyn  Botanic 
Garden.) 

"The  stems,  roots,  and  petioles,  and  even  the  pinnules, 
have  been  found  calcined  and  so  beautifully  preserved 
that  their  entire  structure  can  be  made  out  with  certainty. 
Without  going  into  a  technical  description  of  these  organs, 
it  may  be  said  that  the  stem  when  young,  and  before 
secondary  growth  has  begun,  has  a  very  strong  resemblance 


2O4  HEREDITY   AND   EVOLUTION  IN   PLANTS 

to  the  stem  of  [the  fern]  Osmundq,  but  when  more  mature 

certain   cycadean   characters    appear   to  predominate."1 

Its  foliage  and  other  characters  closely  resemble  some 

of  our  modern  tree-ferns  (Cf.  Figs.  87  and  88),  but  more 


FIG.  88. — Leaf  of  a  fern  (Angiopteris  evecta).     (Cf.  Fig.  87.) 

careful  study  of  the  calcined  specimens  of  much  beauty, 
found  in  calcareous  nodules  (the  so-called  English 
"coal  balls"2),  has  disclosed  both  the  microsporophylls, 

1  Knowlton,  F.  H.     American  Fern  Journal,  5: 85.     1915. 

2  Coal  balls  are  "concretions  of  the  carbonates  of  lime  and  magnesia 
which  formed  around  certain  masses  of  the  peaty  vegetation  as  centers 
and,  through  inclosing  and  interpenetrating  them,  preserved  them  from 
the  peculiar  processes  of  decay  which  converted  the  rest  of  the  vegetation 
into  coal.     In  them  the  mineral  matter  slowly  replaced  the  vegetable 
matter,  molecule  by  molecule,  thus  preserving  the  cellular  structure  to  a 
remarkable  degree.     Such  balls  are  especially  frequent  in  the  coal  of 
certain  parts  of  England  (Lancashire  and  Yorkshire)."     Shimer,  H.  W. 
"  An  introduction  to  the  study  of  fossils,"  p.  53.     London,  1914. 


THE    EVOLUTION   OF   PLANTS 


205 


bearing  pollen-sacs,  and  the  megasporophylls,  bearing, 
not  merely  megasporangia,  but  true  seeds.  The  ovule  has 
a  pollen-chamber,  like  the  cycads,  except  that  it  projects  a 
bit  through  the  micropyle,  and,  strange  as  it  may  seem, 
fossil  pollen-grains  have  been  discovered,  well  preserved 
within  this  chamber.  The  seeds,  about  ^  inch  long, 
have  been  described  as  resembling  little  acorns,  enclosed 
like  hazelnuts  in  smaller  glandular  cupules  (Fig.  89). 
They  are  similar  to  those  of  the  cycads,  except  that  they 
are  not  known  to  have  organized  an  embryo  with  cotyle- 


FIG.  89. — Restoration  of  a  seed  of  Lyginodendron  oldhamium  (Lagenos- 
tema  Lomaxf),  from  a  model  by  H.  E.  Smedley.     (After  Scott.) 

dons  and  caulicle.  Instead,  the  tissues  of  the  female 
gametophyte  only  are  so  far  found,  retained  within 
the  megasporangium,  which  is  enclosed  in  the  integument. 
In  this  connection  it  is  of  interest  to  note  that  the  seeds  of 
some  modern  plants  (e.g.,  .orchids)  do  not  possess  differ- 
entiated embryos,  but  whether  this  is  a  primitive  or  a 
reduced  character  is  not  certain.  The  pollen  was  formed 
in  spindle-shaped  pollen-sacs,  having  two  chambers,  and 
borne  in  clusters  of  four  to  six  on  the  under  side  of  little 
oval  discs,  from  2  to  3  millimeters  long.  These  structures 


2O6  HEREDITY  AND    EVOLUTION   IN  PLANTS 

are  found  on  pinnules  of  ordinary  foliage  leaves,  resem- 
bling the  sporophylls  of  certain  ferns  (Fig.  90)  rather  than 
the  stamens  of  modern  flowers. 

The  discovery  of  the  seed-bearing  character  of  the  fern- 
like  plants  of  the  Paleozoic  has  been  called  the  most  im- 
portant contribution  of  paleobotany  to  botany  ever  made. 
It  was  predicted  by  Wieland,  of  Yale  University,  nearly 
two  years  before  it  was  announced  by  Oliver  and  Scott. 
It  is  now  believed  that  seed-bearing  plants  of  the  pterido- 


FIG.  90. — Top,  lateral  pinna  from  a  leaf  of  Marattia  fraxinea.  (After 
Bitter.)  Below  at  left,  synangium  of  same.  (After  Bitter.)  At  right, 
cross-section  of  the  synangium.  (After  Hooker-Baker.) 

sperm   type  were  nearly  as  numerous  in  the  Paleozoic 
as  were  the  cryptogams. 

140.  Significance  of  the  "Pteridosperms."— The  close 
resemblance  of  the  pteridosperms  to  ferns,  on  the  one  hand, 
and  to  modern  cycads  on  the  other,  justifies  the  conclu- 
sion that  they  represent  a  "connecting  link"  between  the 
true  ferns  and  the  cycads,  and  that  the  modern  cycads 
have  descended  from  the  same  ancestry  as  the  modern 
ferns,  each  developing  along  somewhat  different  lines. 


THE  EVOLUTION  OF  PLANTS 


207 


It  was  in  recognition  of  their  vegetative  resemblances  that 
the  Pteridosperms  were  first  called  (by  Potonie)  Cycado- 
filices,  now  Cycadofilicales.  Van  Tieghem  tersely  de- 
scribed them  as  "phanerogams  without  flowers." 

141.  A  Modern  Fern-like  Cycad.— One  of  the  modern 
cycads  (Stangeria  paradoxa]1  is  of  much  interest  in  this 


FIG.  91. — Stangeria  paradoxa  Moore.      Specimen  from  the  cycad  house 
at  the  New  York  Botanical  Garden,  bearing,  at  the  apex  of  the  stem 
a  carpellate  cone.     (Photo  from  New  York  Botanical  Garden.) 
/ 

connection.  So  closely  does  it  resemble  a  certain  fern 
(Lomaria}  that  the  botanist  Kunze,  who  first  described  it 
when  it  was  brought  from  Natal  to  the  botanic  garden  at 
Chelsea,  England,  supposed  it  was  a  fern,  and  named  it 
Lomaria  eriopus.  The  specimen  possessed  no  fruit,  which 
would  have  helped  to  identify  it.  Its  leaves,  with  circinate 

1  Stangeria  paradoxa  Moore  =  Stangeria  eriopus  (Kunze)  Nash. 


208 


HEREDITY   AND    EVOLUTION   IN   PLANTS 


vernation,  have  a  pinnately  compound  blade,  and  leaflets 
with  pinnate  dichotomous  venation.  Two  or  three  years 
later  another  botanist,  examining  it  more  closely,  pro- 
nounced it  a  "fern-like  Zamia  or  a  Zamia-like  fern." 
These  facts  show  how  puzzling  the  specimen  was,  and  how 


FIG.  92. — To  the  left,  Cacadeoidea  dacotensis  Macbride.  Longitudinal 
section  of  a  silicified  specimen  of  a  bisporangiate  cone  (unexpanded  flower), 
so  taken  that  the  pinnules  of  the  microsporophylls  on  both  sides  of  the 
central  axis,  or  receptacle,  are  successively  cut  throughout  their  entire 
length.  The  lines  indicate  the  planes  of  various  sections  through  the  cone, 
published  in  Wieland's  "American  Fossil  Cycads."  To  the  right  Cycado- 
cephalus  Sewardi  Nathorst.  Microsporangiate  cone,  natural  size,  preserved 
as  an  impression  on  a  flat  slab.  From  a  fossil-bearing  bed  of  the  Trias,  at 
Bjuf,  Southern  Sweden.  (Left  figure  from  Wieland,  right  figure  from 
Nathorst.) 

closely  a  plant  may  resemble  both  acycadophyte  and  a  fern. 
In  a  sense  this  plant  may  be  called  a  living  fossil.  Speci- 
mens have  since  come  into  flower  in  botanic  gardens,  and 
the  typical  cycadaceous  cones  (Fig.  91)  leave  no  doubt 
that  the  plant  is  a  true  cycadophyte. 


THE   EVOLUTION    OF   PLANTS  2OQ 

142.  Derivation  of  New  Types. — Attention  should  here 
again  be  called  to  the  fact  that  the  theory  of  evolution  does 
not  teach  that  one  given  species  becomes  transformed  into 
another,  but  simply  that  new  species  are  descended  from 
older  forms  which  may  or  may  not  continue  to  exist.  It 
is  not  supposed,  for  example,  that  ferns  developed  into 


FIG.  93. — Cycadeoidea  dacolensis.  Semi-diagrammatic  sketch  of  a 
flower  (bisporangiate  cone),  cut  longitudinally;  one  sporophyll  folded,  and 
one  (at  the  right)  arbitrarily  expanded.  At  the  center  is  the  apical,  cone- 
shaped  receptacle,  invested  by  a  zone  of  short-stalked  ovules  and  inter- 
seminal  scales.  The  pinnules  of  the  sporophylls  bear  the  compound 
sporangia  (Synangia).  Exterior  to  the  flower  are  several  hairy  bracts. 
About  three-fourths  natural  size.  (After  Wieland.) 

cycads,  and  cycads  into  higher  gymnosperms,  but  that 
there  has  been  an  unbroken  line  of  descent  (possibly  more 
than  one)  in  the  plant  kingdom,  that  closely  related  forms 
(like  ferns  and  cycads)  have  descended  from  a  common 
ancestral  type  which  may  or  may  not  now  be  found.  We 
must  not,  in  other  words,  expect  necessarily  to  find  in 


210  HEREDITY    AND    EVOLUTION   IN   PLANTS 

fossil  forms  the  direct  ancestors  of  those  now  living,  although 
a  study  of  their  structure  is  of  the  greatest  value  in  ena- 
bling us  to  understand  the  genetic  relationships  of  the  great 
groups  of  plants. 

143.  Ancestors  of  the  Angiosperms. — Just  as  the  Cyca- 
dofilicales  indicate  the  ancestry  of  the  cycads,  so  fossil 
types  of  Cycadophyta  have  been  discovered  which  are 


FIG.  94. — Cycadcoidca  dacotcnsis  (?).  Photomicrograph  of  a  young 
seed  (X  15),  showing  a  sterile  scale  on  either  side.  Between  them  pro- 
jects the  entire  length  of  the  tube  through  which  the  micropyle  extends. 
The  partially  collapsed  nucellus  is  distinctly  shown  in  the  center.  (After 
Wieland.) 

interpreted  by  some  paleobotanists  as  ancestors  of  the 
modern  angiosperms.  Other  investigators,  however, 
dissent  from  this  view  and  consider  that  we  have  not  yet 
sufficient  knowledge  of  fossil  forms  to  be  justified  in  desig- 
nating the  ancestors  of  the  Angiosperms.  This  differ- 
ence of  opinion  is  largely  due  to  the  meagerness  of  the 
available  evidence.  As  one  writer  has  stated  it,  "A 


THE   EVOLUTION   OF   PLANTS 


trayful  of  flowers  may  be  all  the  record  of  the  Pterido- 
sperms  from  the  Devonian  on.  The  gaps  in  the  evidence 
are  always  enormous." 

Although  the   Cycadophyta  are  now  a  very  insignifi- 
cant element  in  the  earth's  flora,  in  the  Mesozoic  period 


FIG.  95. — Macrozamia  spiralis.  Tip  of  the  trunk,  showing  three 
lateral  cones,  inserted  in  the  axils  of  leaves.  Photo  from  specimen  in 
Brooklyn  Botanic  Garden.  (Cf.  Fig.  96.) 

they  form  about  one-third  of  the  recovered  vegetation  of 
the  land.  One  order,  the  Hemicycadales  (Bennettitales1), 
then  had  a  cosmopolitan  distribution  and  seemingly  was 
as  important  as  the  Dicotyledons  are  now.  Overdo  species 
of  the  petrified  stems  have  been  found  in  the  Mesozoic 

1In  his  paper  on  the  Classification  of  the  Cycadophyta  (Am.  Jour.  Sci. 
47:  391-406.  June,  1919),  Wieland  states  "simple  and  good  reasons" 
for  letting  the  name  Bennettitales  fall  into  disuse,  and  substituting  there- 
fore the  term  Hemicycadales  (half-cycads). 


212 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


terrains  of  the  United  States,  the  Black  Hills  of  South 
Dakota  alone  yielding  a  score.     The  Isle  of  Portland  forms 


FIG    96—  Cycas   circinalis.    Tip  of   trunk,   showing   numerous   leaf- 
stalks, and  the  large  terminal  cone.     Photo  from  specimen  in 
Botanic  Garden.     (Cf.  Fig.  95.) 

were  called  Cycadeoidea  by  the  celebrated  geologist  Buck- 
land.     The  original  name  of  the  order  was  derived  from 


THE   EVOLUTION   OF   PLANTS 


213 


FIG.  97. — Cycadeoidea  Wielandi.  At  left,  a  finely  preserved  trunk 
bearing  many  ovulate  cones  with  seeds  approaching  maturity,  and  a  lesser 
number  of  either  young  or  abortive  cones,  j',  Receptacle  of  a  shed  or 
non-preserved  cone  with  surrounding  bracts  yet  present;  f",  two  cones 
broken  away  during  erosion,  with  a  portion  of  the  basal  infertile  pedicel 
yet  remaining;  m,  four  cones  eroded  down  to  the  surface  of  the  armor, 
in  this  instance  about  or  a  little  beneath  the  level  of  the  lowermost  seeds; 
y,  three  of  the  dozen  or  more  very  young  cones,  in  some  cases  known 
to  be  simply  ovulate  and  to  be  regarded  as  having  aborted  or  else  as  be- 
longing to  a  later  and  sparser  series  of  fructifications  than  the  seed-bearing 
cones  present,  the  latter  unquestionably  representing  the  culminant  fruit- 
producing  period  in  the  life  of  this  cycad;  s  (over  lower  arrow),  the  ovulate 
strobilus,  shown  at  the  right,  in  its  natural  position,  this  photograph  having 
been  made  before  the  cone  was  cut  out  by  a  cylindrical  drill.  X  0.5. 

At  right,  longitudinal  section  of  the  small  ovulate  strobilus  cut  from 
its  natural  position  on  the  trunk  as  denoted  by  the  arrow  s,  in  photograph 
i.  c  (upper  arrow),  seed  with  dicotyledonous  embryo  preserved,  cotyle- 
dons being  similarly  present  in  the  lowermost  seed  on  the  left-hand  side 
of  the  strobilus;  s,  traces  of  hypogynous  staminate  disk;  b,  bracts;  /,  leaf 
bases.  X  5.  (After  Wieland.) 


214  HEREDITY  AND   EVOLUTION   IN  PLANTS 

the  genus-name,  Bennettites.1  Other  forms,  usually  found 
as  casts,  are  called  Williamsonia,  still  others  are  known 
mainly  as  genera  founded  on  leaf  imprints. 

144.  Cycadeoidea. — In  most  of  its  purely  vegetative 
characters,  such  as  the  anatomy  of  the  stem  and  the 


FIG.  98. — Cycadeoidea  Wielandi.  Longitudinal  section  through  the 
axis  of  a  female  inflorescence,  or  cone.  /,  old  leaf-base;  d,  insertion  of 
disc;  s,  erect  seed,  borne  at  summit  of  seed-pedicle  inserted  on  convex 
receptacle;  b,  hair-covered  bract.  (After  Wieland.) 

structure  of  the  leaves,  Cycadeoidea  resembled  modern 
cycads,  but  its  reproductive  branches  were  character- 
istically lateral,  which  is  one  of  the  most  fundamental 
characteristics  of  the  higher  seed-bearing  plants  of  to- 
day. Only  two  modern  cycads  (Macrozamia  and  Bow- 

1  Cycadeoidea  Buckland  =  Bennettites  Carruthers. 


THE    EVOLUTION   OF   PLANTS 


215 


enia)  have  lateral  seed-bearing  cones  (Fig.  95);*  in  the 
other  genera  the  carpellate  cones  are  terminal  (Fig.  96) . 
Various  structural  characters  of  Cycadeoidea  are  shown 
in  Figs.  92-100. 

In  Cycadeoidea  dacotensis  the  "flower,"  which  in  some 
specimens  was  5  inches  long,  was  a  strobilus,  consisting  of 
a  thick  axis  on  the  lower  part  of  which  were  numerous 


If 


FIG.  99. — Cycadeoidea  ingens.  Restoration  of  an  expanded  bispor- 
angiate  cone,  or  flower,  in  nearly  longitudinal  section.  Restored  from  a 
silicified  fossil.  (After  Wieland.) 

bracts  arranged  in  spirals.  The  bracts  surrounded  a 
campanula  of  about  20  stamens.  Each  stamen  was,  in 
reality,  a  pinnately  compound  sporophyll,  about  4  inches 
long,  rolled  in  toward  the  center  of  the  flower,  and  bear- 
ing two  rows  of  compound  microsporangia  (pollen-sacs) 
on  each  leaflet.  They  thus  closely  resembled  the  sporo- 
phyll of  a  fern. 

1  The  staminate  cones  of  Zamia  are  lateral. 


216 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


The  axis  of  the  flower  terminated  in  a  cone-shaped 
receptacle,  bearing  the  stalked  ovules,  and  numerous 
sterile  scales  (Figs.  97  and  98).  The  mature  seeds  often 
contain  the  well-preserved  fossil  embryos,  with  two 
cotyledons  which  quite  fill  out  the  nucellus,  and  show 
that  there  was  little  or  no  endosperm.  These  are  char- 
acters never  found  in  the  lowest  group  of  modern  seed- 


FIG.  ioo. — Cycadeoidea  Dartoni.  Tangential  section  through  outer 
tissues  of  the  (fossilized)  trunk,  showing  the  very  numerous  seed-cones. 
The  seeds  are  very  small  (the  illustration  being  natural  size),  and  nearly 
every  one  has  a  dicotyledonous  embryo.  There  were  over  500  such  cones 
on  the  original  stem.  (After  a  photograph  loaned  by  Prof.  Wieland.) 

bearing  plants  (the  Gymnosperms),  but  only  in  the 
highest  group  of  Angiosperms,  the  Dicotyledons.  In 
fact,  the  French  paleobotanist,  Saporta,  called  some  of  the 
Cycadeoids,  Proangios perms. 

145.  Relation  of  Cycadeoidea  to  Modern  Angiosperms. 
• — The  question  of  the  ancestry  of  the  Angiosperms  is  the 
most  important  problem  of  paleobotany.  Although  the 


THE  EVOLUTION  OF  PLANTS  217 

Hemicycadales  possess  many  of  the  primitive  anatomical 
features  that  characterize  the  Cycadofilicales,  their 
development  of  a  bisporangiate  strobilus  with  two  sets 
of  sporophylls,  related  to  one  another  as  they  are  in  the 
flower  of  the  Angiosperms,  indicates  a  genetic  relationship 
to  that  group,  as  does  also  the  fact  that  the  seeds,  enclosed 
in  a  fruit,  possess  a  dicotyledonous  embryo,  without  endo- 


FIG.  101. — Flower  of  magnolia.     (Cf.  Fig.  102.) 

sperm.  In  other  features  the  Hemicycadales  are  unlike 
the  Angiosperms;  the  ovules,  for  example,  are  enclosed 
by  sterile  scales,  instead  of  by  the  carpels  on  which  they 
are  borne,  and  the  protrusion  of  the  pollen-chamber 
through  the  micropyle  signifies  the  gymnospermous  type  of 
fertilization. 

These  and  other  comparisons  indicate  that  the  Hemi- 
cycadales were  essentially  Gymnosperms  having  certain 


2l8  HEREDITY   AND   EVOLUTION   IN   PLANTS 

Angiospermous  characters,  and  therefore,  while  they  are  not 
to  be  considered  as  the  ancestors  of 
the  Angiosperms,  it  is  probable  that 
they  and  the  modern  dicotyledons 
are  both  descended  from  a  common 
branch  of  the  ancestral  tree.  Among 
modern  plants,  the  flower  of  the 
magnolias  most  closely  resembles 
that  of  Cycadeoidea  in  the  spiral 
arrangement  of  its  stamens  and 
pistils  (Figs.  101  and  102).  Just 
what  significance  should  be  attached 
to  that  fact  has  been  disputed  by 
students  of  morphology.  The  older 
view  of  the  systematists  regarded  the 
primitive  flower  as  more  complex  in 
structure,  with  pistils,  stamens,  and 
floral  envelopes  arranged  spirally  in 
centripetal  or  acropetal  succession  on 
a  fleshy  axis,  as  in  Magnolia  and  other 
flowers  of  the  order  Ranales;  other 
types  of  floral  structure  were  con- 
FIG.  io2.-Magnolia  sidered  as  derived  from  this  one  by 

Flower    with     perianth  . 

removed,   showing    the  reduction.     This  is  often  referred  to 

compound  pistil,  and  four  as   the    "Strobiloid    theory   of   the 

ofthestamens.     Mostof  flower  •"  (Cf.  pp.  132  and  I34). 

the  stamens  have  been 

removed  so  as  to  bring  A    more     recent     V1CW    recognizes 

out  their  spiral  arrange-  that  simple  staminate  or   pistillate 
ment  as  shown  by  the  flowers  may,  in  some  cases,   be  in- 

scars    at   the   points   of  ^    &&    derived    b       reduction 

attachment.       (Cf.    Fig.  * 

IOI>)  from  more  complex  forms,  but  re- 

gards the  primitive  flower  as  uni- 


THE    EVOLUTION   OF   PLANTS 


2IQ 


sexual — in  effect  a  microsporophyll  or  a  megasporophyll, 
from  which  complex  forms  were  derived  by  elaboration. 
This  latter  view,  however,  is  not  in  harmony  with  avail- 
able evidence  from  fossil  plants,  such  as  that  afforded  by 
Cycadeoidea. 

"  The  strobiloid  theory  of  the  flower  seems  in  the  present 
state  of  our  knowledge  to  stand  alone  as  a  working  hy- 


FIG.  103. — Theoretical  stages  in  the  reduction  (from  Cycadeoidea  to 
modern  Angiosperms)  of  staminate  discs  represented  as  segments.  A, 
any  common  campanulate  form  with  simple  stamens  (e.g.,  morning 
glory);  B,  hypothetical  Cycadeoid  reduced  to  a  single  synangium  to  each 
frond  component;  C,  inner  view  of  a  sector  of  a  Williamsonia  mexicana 
disc;  D,  sector  of  a  Cycadeoidea  dacotensis  disc  with  the  pair  of  shoulder 
spurs  borne  by  each  frond.  (After  Wieland.) 

pothesis.  If  we  reject  it,  we  are  left  without  any  historical 
clue  to  the  origin  of  the  floral  structure  of  Angiosperms. 
If  we  accept  it,  the  Primitive  Angiosperm  must  be  cred- 
ited with  a  flower  resembling  that  of  Magnolia  or  Lirio- 
dendron  in  general  plan."1  From  this  it  follows  that  the 
Magnoliacea  must  be  among  the  most  primitive,  if  not  the 
most  primitive,  of  all  Angiosperms,  as  Wieland  first  and 
Hallier  later  and  independently  pointed  out. 

1  Sargant,  Ethel.  The  reconstruction  of  a  race  of  primitive  Angiosperms. 
Ann.-Bot.  22:121-186.  April,  1908. 


22O  HEREDITY   AND   EVOLUTION   IN   PLANTS 

The  gap  between  the  stamen  of  Cycadeoidea  and  the 
type  characteristic  of  modern  Angiosperms  is  partially 
bridged  by  the  genus  Williamsonia  (which  has  simple  vs. 
pinnately  compound  stamens),  and  by  another  genus, 
Wielandiella,  both  older  genera  than  Cycadeoidea  (Fig. 
103).  From  this  it  has  been  inferred  that  the  Hemicy cad- 
ales  are  a  lateral  branch,  further  removed  than  their 
ancestors  from  the  direct  evolutionary  stock  of  the 
Angiosperms. 

146.  Origin  of  Dicotyledony. — Two  problems  of  major 
importance  are  involved  in  the  question  of  the  evolution 
of  Angiosperms,  namely,  the  origin  of  dicotyledony  and 
the  origin  of  monocotyledony.  Are  dicotyledons  more 
ancient  than  monocotyledons,  or  vice  versa?  Again,  in 
the  evolution  of  seed-bearing  plants  was  the  condition  of 
polycotyledony  antecedent  to  that  of  dicotyledony,  or  the 
reverse?  This  would  be  a  comparatively  easy  question 
to  answer  if  we  had  an  unbroken  series  of  fossil  remains 
of  the  primitive  and  intermediate  spermatophy tes ;  but 
unfortunately  such  evidence  has  not  yet  been  discovered. 
We  know  nothing  of  the  embryos  of  the  geological  ances- 
tors of  modern  conifers.  The  Mesozoic  gymnosperms 
(Cycadeoidea  and  other  related  genera)  are  known  to  have 
had  dicotyledonous  embryos,  but  these  forms  do  not  stand 
in  the  ancestral  line  of  the  (polycotyledonous)  conifers 
of  to-day.  To  answer  our  question,  therefore,  we  must, 
for  the  present,  depend  largely  on  the  study  of  living  forms. 
The  evidence  has  seemed  conflicting,  and  for  nearly  three- 
quarters  of  a  century  opinion  has  varied.  Adanson  and 
Jussieu,  in  the  early  nineteenth  century,  contended  that 
polycotyledony  was  derived  from  dicotyledony  by  a  split- 
ting of  the  primordia  of  two  original  cotyledons;  Sachs 


THE   EVOLUTION    OF   PLANTS 


(1875)  held  the  opposite  opinion.  Hill  and  de  Fraine 
(1908-1910)  are  among  the  recent  protagonists  for  the 
hypothesis  that  dicotyledons  are  the  more  primitive.  One 
of  the  most  recent  studies  is  that  by  Bucholz1  who  ex- 


L  I  K  J  G 

FIG.  104. — Development  of  stem  tip  and  cotyledons  in  Finns  Bank- 
siana.  Dotted  line  represents  plerome  of  root-tip;  shaded  area,  meristem 
of  stem  tip;  H,  I,  J,  K,  fusing  cotyledons.  (After  Buchol/.) 

amined  the  embryos  of  pine,  spruce,  larch,  jumper,  balsam 
fir,  cedar  of  Lebanon,  and  others.  Many  instances  of 
the  fusion  of  the  primordia  of  cotyledons  were  found,  but 
no  evidence  of  cotyledonary  splitting.  This  fusion  has 
resulted  in  reducing  the  number  of  cotyledons,  and,  in 

1  Bucholz,  John  T.  Studies  concerning  the  evolutionary  status  of  poly- 
cotyledony.  Am.  Journ.  Bot.  6  : 106-119.  March,  1919. 


222  HEREDITY   AND    EVOLUTION    IN   PLANTS 

certain  species,  in  the  formation  of  a  cotyledonary  ring, 
or  tube.  Bucholz  interprets  the  facts  set  forth  by  him- 
self and  other  investigators  as  leading  to  the  conclusion 
that  the  more  primitive  gymnosperms  had  numerous  coty- 
ledons, that  their  number  was  reduced  by  the  fusions  of 
their  primordia  and,  in  some  species,  a  cotyledonary  tube 
or  ring  was  formed.  "Dicotyledony  was  attained  either 
by  a  general  fusion  of  many  cotyledons  in  two  groups,  or 


Fig.  105. — Polycotyledonous  seedlings  of  dicotyledonous  species.  A-C, 
Silene  odontipetala,  with  hemi-tricotylous,  tricotylous,  and  tetracotylous 
seedlings;  D-H,  Papaver  Rhaeas  (semi-double  cultivated  form),  dicoty- 
lous,  hemi-tricotylous,  tricotylous,  tetracotylous,  and  pentacotylous 
seedlings;  /,  Acer  Pseudo-Plat  amis,  tetracotylous  seedling.  (All  figures 
re-drawn  from  de  Vries.) 


by  an  extremely  bilabiate  development  of  a  cotyledonary 
tube"  (Fig.  104). 

The  final  conclusion  of  Bucholz,  based  on  the  evidence 
of  comparative  anatomy,  supplemented  by  studies  of 
development,  is  that  the  polycotyledonous  condition  is 
the  more  primitive,  and  the  dicotyledonous  one  derived. 
On  the  basis  of  this  theory,  the  rather  common  abnormal 
a  ppearance  of  supernumerary  cotyledons  in  dicotyledonous 


THE    EVOLUTION    OF   PLANTS  223 

seeds  is  to  be  interpreted  as  a  reversion  to  a  more  primitive 
condition  (Fig.  105). l 

147.  Origin  of  Monocotyledony. — If  the  earliest  Angio- 
sperms  were  dicotyledons,  as  now  seems  probable,  the 
monocotyledons  were  probably  derived  from  them  by 
a  process  of  simplification.  Several  hypotheses  have  been 
framed  as  to  how  the  final  result  was  accomplished,  but 
the  voluminous  evidence  and  the  conclusions  can  only  be 
briefly  summarized  here. 

For  nearly  a  century  it  has  been  generally  accepted 
by  botanists  that  the  two  seed-leaves  or  cotyledons  of 
dicotyledonous  plants  were  lateral  organs,  originating 
below  the  tip  of  the  embryonic  stem  or  hypocotyl,  while 
the  single  cotyledon  of  monocotyledonous  plants  was 
considered  as  a  terminal  organ.  The  grass  family  offers 
a  case  in  point.  The  embryo  of  Indian  corn  (Zea  Mays}, 
for  example  possesses  a  well  developed  cotyledon,  called 
the  scutellum;  there  is  little  or  no  trace  of  a  second 
cotyledon.  The  embryos  of  many  other  grasses,  however, 
possess  an  organ,  the  epiblast,  homologous  in  position  with 
the  scutellum,  and  regarded  by  earlier  botanists  as  a  rudi- 
mentary cotyledon  (Fig.  106).  Recent  studies  of  Coulter 
and  Land  leave  little  doubt  of  this  as  the  correct  interpre- 
tation of  that  organ. 

A  study  by  Bruns  (1882)  of  82  genera  of  grasses,  repre- 

1  According  to  de  Vries  (The  Mutation  theory.  2:393-456.  Chicago, 
1910)  tricotylous  intermediate  races  do  not  arise  by  selection  but  by 
mutation,  tricotyly  being  the  expression  of  an  ancestral  character  which 
is  latent  in  the  normal  species.  If  the  normal  character  is  active  and 
the  anomaly  semi-latent  we  have  what  de  Vries  calls  a  "half-race;"  if 
the  normal  character  becomes  latent  and  the  anomaly  active,  we  have  a 
"constant  variety."  Sometimes  an  equilibrium  is  maintained  in  the  ex- 
pression of  the  normal  character  and  the  anomaly,  giving  rise  to  a 
"middle  race,"  or  "eversporting  variety." 


224 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


senting  12  tribes,  demonstrated  the  presence  of  the  rudi- 
mentary cotyledon  (epiblast)  in  29  of  the  genera,  repre- 


FIG.  106. — Diagram  of  longitudinal  sections  of  grass-embryos  (Gram- 
ineae)  to  illustrate  the  rudimentary  cotyledon  (epiblast).  A-C,  \Er-G, 
redrawn  from  J.  M.  Coulter,  after  Bruns;  D,  from  nature.  A,  Zizania 
aquatica;  B,  Leersia  clandestine^;  C,  Leptochloa  arabica;  D,  Triticum  vul- 
gare;  E,  Spartina  cynosuroides;  F,  Triticum  vulgare;  G,  Zea  Mays; 
s,  scutellum;  c,  coleoptile;  e,  epiblast. 

senting  nine  tribes;  later  studies  by  Van  Tieghem  (1897) 
disclosed  the  presence  of  an  epiblast  in  61  out  of  91  genera 
examined.  From  these  figures  we  may  reasonably  infer 


THE    EVOLUTION   OF   PLANTS  225 

that  the  majority  of  the  so-called  "  monocotyledonous " 
grasses  possess  two  cotyledons,  one  of  which  is  more  or  less 
rudimentary,  and  that  the  grasses  are  primitive  monocoty- 
ledons, representing  a  transitional  stage  from  dicotyledons 
to  the  higher  monocotyledons.  Monocotyledony,  then,  as 
stated  by  Coulter,  is  simply  one  expression  of  a  process 
common  to  all  cotyledony,  gradually  derived  from  dicotyle- 
dony  by  reduction,  and  involving  no  abrupt  transfer  of  a 
lateral  organ  to  a  terminal  origin.  Variations  in  the  rela- 
tive size  of  the  second  cotyledon  in  grass  embyros  are 
illustrated  in  Fig  106. 

Henslow1  was  among  the  first  to  suggest  the  origin  of 
monocotyledons  from  dicotyledons. 2  Previous  to  the  pub- 
lication of  his  paper,  it  was  generally  assumed  that  mono- 
cotyledons were  the  older  group,  and  Henslow  stated  that 
no  systematist  of  his  day  recognized  any  real  points  of  con- 
nection between  the  two  groups.  He  proposed  the  hy- 
pothesis that  the  monocotyledons  were  derived  by  the 
arrest  of  the  development  of  one  seed-leaf  in  a  primitive 
dicotyledonous  Angiosperm;3  hence  said  Henslow,  "only 
one  elongates,  its  superior  vigour  carrying  it  on  in  a  straight 

1  Henslow,  Rev.  George.     A  theoretical  origin  of  endogens  from  exogens, 
through  self-adaptation  to  an  aquatic  habit.     Journ.  Linnean  Soc.  Bot. 
19  : 485-528.     May  15,  1893. 

2  The  first  to  make  the  suggestion  appears  to  have  been  Agardh,  in  his 
Larobok  i  Botanik,  Part  I.     Malino,  1829-32. 

3  In  discussing  the  origin  of  Angiosperms,  Arber  (Journ.  Linnean  Soc. 
Bot.  38  : 29-80.    July,  1907)  calls  attention  to  the  "Law  of  corresponding 
stages  in  evolution,"  namely,  that  in  the  evolution  of  seed-plants,  the  stages 
reached  by  different  organs  at  any  one  period  are  dissimilar.     From  this 
law  it  follows  that  such  a  plant  as  a  "primitive  Angiosperm,"  in  the  strict 
sense  of  the  term,  that  is,  with  all  its  organs  primitive,  never  existed  in 
reality.     We  must  picture  the  ancestors  of  modern  Angiosperms  as  having 
certain,  organs  in  a  primitive  stage  of  evolutionary  development,  others 
as  more  advanced  toward  the  stage  in  which  they  are  now  found. 

is. 


226  HEREDITY   AND   EVOLUTION   IN   PLANTS 

line  with  the  suspensor,  finally  making  the  cotyledon  ter- 
minal." This  he  calls  "the  real  interpretation  of  a  mono- 
cotyledonous  embryo."  Henslow  further  inferred  a  very 
early  origin  of  monocotyledons  from  dicotyledons,  from 
the  fact  that  so  many  of  their  orders  contain  very  few  gen- 
nera  and  monotypic  groups,  for  groups  of  plants  or  animals 
with  few  members,  are  regarded,  in  general,  as  survivals, 
representing  a  lost  ancestry.  He  recorded  voluminous 
observations  in  support  of  his  theory,  and,  among  other 
evidence,  called  attention  to  "Dicotyledonous  monocoty- 
ledons "  such  as  Tamus  communis  (black  bryony),  a  tuberous 
rhizomed  species  of  the  Yam  family,  where  the  first  foliage 
leaf,  situated  exactly  opposite  the  cotyledon,  is  interpreted 
(with  Dutrochet)  as  a  second  cotyledon;  and  to  "Mono- 
cotyledonous  dicotyledons,"  especially  among  aquatic 
species  such  as  the  water-chestnut  (Trapa  natans),  where 
one  cotyledon  is  arrested  in  its  development.  Other  illus- 
trations, not  mentioned  by  Henslow,  include  such  forms 
as  Dioscorea  bonariensis  and  Pinguicula  vulgaris  (Fig.  107). 
Ranunculus  Ficaria  is  not  an  aquatic,  but  it  nourishes 
by  the  waterside,  and  is  regarded  by  Henslow  as  descended 
from  an  aquatic  form.  About  one-third  of  the  orders  of 
of  monocotyledons  are  aquatic,  as  compared  to  only  4 
per  cent,  in  dicotyledons,  and  the  monocotyledonous  dicoty- 
ledons are  all  aquatic.  The  final  conclusion  of  Henslow 
is,  "that  endogens  [monocotyledons]  have  in  the  first  place 
descended  from  very  early  types  of  exogens  [dicotyledons] 
...  ;  and  that,  secondly,  the  more  immediate  cause  of 
their  origin  was  an  aquatic  habit  of  life  assumed  by  certain 
primitive  exogenous  plants." 

Miss  Ethel  Sargant  has  more  recently  elaborated  the 
hypothesis   of   the   derivation   of   monocotyledons   from 


THE   EVOLUTION    OF   PLANTS 


227 


dicotyledons,  by  a  fusion  of  the  two  cotyledons  into  one. l 
On  this  basis  the  single  seed-leaf  of  monocotyledons  is 
interpreted  as  homologous  to  the  two  seed-leaves  of  di- 
cotyledons. The  evidence  supporting  this  suggestion  is 
derived  largely  from  a  study  of  the  anatomy  of  monocoty- 
ledonous  seedlings.  ''The  young  epicotyl  of  monocotyle- 
donous  seedlings  contains  a  single  ring  of  collateral  bundles 
which  may  even  show  traces ~of  cambium,  much  resembling 


FIG.  107. — A-R,  embryos  of  a  "dicotyledonous  monocotyledon,"  A, 
longitudinal  section  through  an  embryo  of  Tamus  commnnis;  B,  Tamus 
communis,  entire  (A  and  B  enlarged  after  Solms-Laubach.).  C-G, 
embryos  of  "monocotyledonus  dicotyledons;"  C,  D,  Dioscorea  bonariensis, 
enlarged  (after  Beccari);  E,  Trapa  natans,  the  water  chestnut  X  M 
(after  Barneoud);  F,  Pinguicula  vulgaris;  G,  Pingmcula  caudata.  (F  and 
G  after  Dickson,  both  greatly  enlarged.) 

dicotyledons."  Professor  Jeffrey  has  also  called  attention 
to  evidence  that  the  anatomy  of  the  stem  of  the  hypotheti- 
cal ancestor  of  the  Angiosperms  was  exogenous  (dicotyle- 
donous). 

Miss  Sargant  has  further  pointed  out  that  the  few 
dicotyledons  which  possess  but  one  seed-leaf  (pseudo- 
monocots)  are  widely  distributed  through  the  dicotyle- 
donous families,  from  Ranunculaceae  to  Umbilliferae, 

1  Annals  of  Botany  17:  1-88.  Jan.,  1903;  Botanical  Gazette  37:  325- 
345.  -May,  1904,  and  other  papers. 


228  HEREDITY   AND    EVOLUTION   IN   PLANTS 

Primulaceae,  and  Nyctaginaceae,  which  indicates  that  the 
abnormality  was  not  derived  by  inheritance  from  a  com- 
mon ancestor;  its  explanation,  therefore,  must  be  sought 
in  the  influence  of  environment.  Professor  Henslow,  as 
noted  above,  associated  the  monocotyledonous  tendency 
with  an  aquatic  habit  of  life,  but  Miss  Sargant  points  out 
that  all  the  pseudo-monocots  possess  some  underground 
organ  which  is  thickened  as  a  tuber,  suggesting  that  the  signi- 
ficant ecological  factor  is  a  geophilous,  rather  than  aquatic, 
habit.  In  further  confirmation  Miss  Sargant  notes  that 
of  twenty  genera  having  their  seed-leaves  fused  for  some 
distance  upward  from  the  base,  the  majority  have  a 
tuberous  hyocotyl.  The  dicotyledonous  may-apple  (Po- 
dophyllum),  for  example,  with  a  geophilous  habit  has 
partially  united  cotyledons  and  a  stem  anatomy  resembling 
that  of  the  monocots.  The  only  exception  to  correlation 
of  this  nature  is  the  mangrove  (Rhizophora  Mangle}, 
a  tropical  tree  whose  seeds  germinate  in  the  air  while  still 
in  the  fruit. 

The  monocotyledons  are  separated  from  the  dicotyle- 
dons by  seven  characters  as  follows:1 

1.  A  single  cotyledon. 

2.  Stem-anatomy. 

3.  Development  of  the  embryo. 

4.  Parallel  venation  of  leaves. 

5.  Short  duration  of  primary  root. 

6.  Seeds  with  endosperm. 

7.  Parts  of  the  flower  in  threes. 

Of  these  characters,  "four  have  been  shown  to  appear 
frequently  among  geophytes,  and  to  be  useful  to  the  plant 
growing  under  conditions  which  determine  the  geophilous 
1  As  enumerated  by  Miss  Sargant. 


THE    EVOLUTION   OF   PLANTS  22Q 

habit.  They  are  therefore  in  all  probability  adaptations 
to  that  habit.  Two  more — stem  anatomy  and  the  ap- 
parently terminal  cotyledon  in  the  embryo — may  be 
considered  as  direct  consequences  of  such  adaptations; 
the  stem  anatomy  acquiring  its  peculiar  features  from  the 
insertions  of  numerous  broad-based  leaves  on  a  squat 
subterranean  axis,  and  the  embryonic 
cotyledonary  number  arising  from  the 
congenital  fusion  of  two  ancestral  cotyle- 
dons. The  seventh  character — trimerous 
floral  symmetry — bears  no  obvious  re- 
lation to  the  geophilous  habit,  but  is  not 
inconsistent  with  it." 

Recent  evidence  as  to  how  monocoty- 
ledony  may   have   been    derived   from 
dicotyledony  has  been  furnished  by  a 
study  of  the  embryogeny  of  Agapanthus      FiG      x o g  _ 
umbellatus  L'Her    (Fig.    108),   a   South    Agapanthus 
African  plant  of  the  Lily  family.  umbellatus.    A, 

The  sequence  of  events  is  as  follows.1  m  o  n  o  cotyledonous 

.  ,       embryo.    Z>,dicoty- 

As  the  massive  pro-embryo  enlarges  the  iedonous     embryo. 

root-end  elongates,  thus  remaining  (Redrawn  from 
narrow  and  pointed;  while  the  shoot-end  Photo  by  w- J-  G- 
widens,  becoming  relatively  broad  and 
flattish.  At  this  broad  and  flat  end  the  peripheral  cells 
remain  in  a  state  of  more  active  division  than  do  the 
central  cells,  and  form  what  is  known  as  the  cotyledonary 
zone.  In  this  zone  two  more  active  points  (primordia) 
appear  and  begin  to  develop.  Soon  the  whole  zone  is 
involved  in  more  rapid  growth,  resulting  in  a  ring  or 

1  The  above  description  closely  follows  Coulter  and  Land.     The  origin 
of  monocotyledony.    Bot.  Gaz.  57:  509-518.     June,  1914. 


230  HEREDITY   AND   EVOLUTION   IN   PLANTS 

tube,  but  with  the  primordia  still  evident.  The  cotyle- 
donary  zone  continues  its  growth  until  a  tube  of  con- 
siderable length  is  developed,  leaving  the  apex  of  the  pro- 
embryo  depressed.  At  this  stage  either  one  of  two  things 
may  occur.  As  the  cotyledonary  zone  continues  to  grow, 
the  two  primordia  on  the  rim  of  the  tube  may  continue 
to  develop  equally,  forming  two  cotyledons;  or  one  of  the 
primordia  may  cease  to  grow,  resulting  in  an  embryo  of 
only  one  cotyledon;  in  other  words,  the  entire  cotyledo- 
nary zone  may  develop  under  the  guidance  of  only  one 
growing  point.  It  is  not  that  one  cotyledon  is  eliminated, 
but  the  whole  growth  is  diverted  into  one.  There  thus 
develops  what  appears  to  be  an  "open  sheath"  and  a 
"terminal"  cotyledon. 

In  other  words,  according  to  Coulter  and  Land,  mono- 
cotyledony  is  not  the  result  of  the  fusion  of  two  cotyledons, 
nor  of  the  suppression  of  one;  but  is  simply  the  con- 
tinuation of  one  growing  point  on  the  cotyledonary  ring, 
rather  than  a  division  of  the  growth  between  two  growing 
points.  In  a  similar  way,  polycotyledony  is  the  appear- 
ance and  continued  development  of  more  than  two  growing 
points  on  the  cotyledonous  ring  (C/.  p.  222,  and  Fig.  104). 

We  are  not  in  possession  of  enough  facts  to  construct 
a  genealogical  tree  showing  the  derivation  of  Mono- 
cotyledons from  Dicotyledons,  nor  the  derivation  of  the 
original  Angiosperm  stock,  but  the  table  of  Arber  and 
Parkin  (Table  IV,  p.  231)  shows  in  a  very  general  pro- 
visional way  a  possible  course  of  events,  and  the  ap- 
proximate geological  period  when  the  various  advances 
were  made,  beginning  with  the  Paleozoic  Cycadofilices 
(Pteridosperms). 

The  first  step  in  the  immediate  evolution  of  the  Angio- 


THE    EVOLUTION   OF   PLANTS 


231 


TABLE  iv 

(After  Arber  and  Parkin) 


Tertiary 


Mesozoic 


Ranalian  plex  us 


Eu-anthostrobilateae 
Pro-anthostrobilati'ae 


Monosporangiateae 


I      Ar 


iphisporangiateae 


sperms,  according  to  Arber,1  was  the  transfer  of  the  pollen- 
collecting  surface  from  the  ovule  to  the  carpel  or  carpels, 
resulting  in  the  stigma  as  now  known.  "It  was  this  act 
which  called  the  Angiosperms  into  being." 

Arber  does  not  regard  the  Apetalous  orders  (Piperales, 
Amentiferous  families,  and  Pandanales)  as  primitive 
Angiosperms,  for  that  theory  necessitates  the  view  that 
the  perianth  arose  de  now,  by  enation.2  He  considers 

1  Arber,  E.  A.  Newell.  On  the  origin  of  Angiosperms.  Jour.  Linnean. 
Soc.  Bot.  38:  29-80.  July,  1907. 

2C/.  p.  132. 


232  HEREDITY  AND    EVOLUTION  IN   PLANTS 

the  perianth  an  ancient  structure,  present  in  the  ancestors 
of  the  Angiosperms,  and  inclosing  an  axis  ("  amphispor- 
angiate  cone")  bearing  both  megasporophylls  and  micro- 
sporophylls.  Such  a  structure  is  called  by  Arber  an 
"anthostrobilus"  The  term  "flower,"  should  be  re- 
stricted to  Angiosperms,  and  may  be  termed  an  "eu- 
anthostrobilus."  The  earlier  form  of  anthostrobilus  (such 
as  occurs  in  modern  Gymnosperms,  and  in  the  Mesozoic 
Benettiteae)  is  called  a  pro- anthostrobilus.  The  hypo- 
thetical, direct  ancestors  of  the  Angiosperms  are  called 
"  Hemiangiospermce"  and  the  possible  order  of  evolu- 
tionary development  is  conceived  by  Arber  as  follows: 

J  Mesozoic  and  Tertiary  (Recent) 
5.  Angiospermae  1  _          .          ... 

I  Eu-anthostrobilatas. 

[  Mesozoic — Pro-anthostrobilatae. 
(rossils  unknown)  J 

3.  Cycadofilices 

2.  Heterosporous  fern-like  \ 

ancestor  I  Paleozoic — Non-strobilate 

i.  Homosporous  fern-like  (    ancestors. 

ancestor  J 

148.  Ancestors  of  the  Gymnosperms.— As  far  back  as 
Devonian  time,  preceding  the  great  coal  period  (Carbon- 
iferous), fossils  have  been  found  of  a  plant,  Cordaites  (of 
the  order  Cordaitales),  common  in  that  period,  and 
having  characters  which  indicate  that  it  stands  in  the 
ancestral  line  of  our  modern  conifers — that  it  and  the 
conifers  had  a  common  ancestry. 

The  leaves  of  Cordaites  resembled  those  of  the  Kauri 
pines  (Agathis]  of  the  southern  hemisphere  (Fig.  109), 
or  the  leaflets  of  Zamia.  They  varied  from  a  decimeter  to 
over  a  meter  in  length.  The  male  cones  resembled  those 
of  the  still  living  Ginkgo,  each  stamen  having  from  four 


THE    EVOLUTION   OF   PLANTS  233 

to  six  microsporangia  (pollen-sacs)  on  a  stalk.  The  female 
cones  resembled  the  male  in  general  appearance,  and 
the  seeds  resembled  those  of  the  Cycadofilicales  (Fig.  94). 
The  plant  itself  was  a  slender  tree,  some  forms  of  which 
attained  a  height  of  over  100  feet.  The  Cordaitales 
formed  the  world's  first  great  forests.  They  represent  a 


FIG.  109. — Branch,  with  cones,  of  the  Kauri  pine  (Agathis  australis). 
(From  the  Gardener's  Chronicle.) 

wide  departure  from  the  Cryptogams,  and  must  be  con- 
sidered as  true  seed-bearing  plants.  They  were  closely 
related  to  the  Ginkgo — another  living  fossil,  ranking  next 
below  the  modern  cone-bearing  trees.  We  thus  ascend 
from  the  ferns  to  the  conifers  by  a  series  of  transitional 
forms  as  follows  (reading  from  the  bottom,  up) : 


234  HEREDITY   AND    EVOLUTION   IN   PLANTS 

6.  Coniferales  (modern  cone-bearing  trees). 

5.  Ginkgoales  (primitive  gymnosperms). 

4.  Cordaitales  (transitional  conifers). 

3.  Cycadales  (true  cycads). 

2.  Cycadofilicales  (cy cad-like  ferns). 

i.  Filicales  (true  ferns). 

149.  Relation  of  the  Above  Groups. — It  must  not  be 
inferred  that  the  above  groups  were  derived  one  from  the 
other  by  descent  from  lower  to  higher.     They  should  be 
interpreted  rather  as  samples  remaining  to  show  us,  not 
the  steps,  but  the  kinds  of  steps  through  which  the  plant 
kingdom  has  passed  in  developing  the  more  highly  organ- 
ized, modern  cone-bearing  trees  from  more  primitive  forms 
like  the  ferns.     As  stated  above,  it  is  doubtful  if  the  actual 
transitional  forms  have  been  preserved,  so  that  the  entire 
history  of  development  can  probably  never  be  written. 

150.  A  Late  Paleozoic  Landscape. — The  frontispiece 
illustrates   the  kind  of  landscape  that  must  have  been 
common  in  the  latter  part  of  the  Paleozoic  era  along 
sluggish  streams  in  certain  regions  such  as  Texas  and  New 
Mexico.     Of  the  primitive  vertebrates  then  abounding, 
only  a  few  larger  types  are  shown.     The  dragon-flies  of  that 
time  are  known  to  have  had  a  spread  of  wing  amounting, 
in  some  cases,  to  as  much  as  two  feet.     In  the  foreground, 
at  the  left,  are  representatives  of  the  Cycadofilicales,  some 
of  them  bushy,  and  others  resembling  our  modern  tree  ferns. 
At  the  right  are  dense  thickets  of  Calamites,  the  ancient 
representatives  of  our  modern  scouring  rushes  (Equisetum). 
In   the   background,    at   the   left,    are   the   unbranched, 
Sigillarias,  and  the  branched  Lepidodendrons.     The  Cor- 
daitales, which  formed  the  Devonian  forests,  were  not  yet 
extinct,  but  none  is  shown  in  the  figure.     Other  forms, 


THE    EVOLUTION   OF   PLANTS  235 

ancestors  of  our  modern  conifers  and  angiosperms,  must 
be  imagined  as  hidden  in  the  recesses  of  the  forest. 

151.  Significance  of  the  Fossil  Record. — Before  the 
brilliant  discoveries  in  fossil  botany,  just  outlined,  were 
made,  there  had  been  (as  stated  in  Chapter  VI)  a  general 
tendency  among  botanists  to  consider  the  comparatively 
simple  moss-plants  as  an  older  type  than  the  fern,  and 
that  either  they  or  their  close  relatives  were  the  ances- 
tors of  Pteridophytes.  As  outlined  in  the  same  chapter, 
the  sporogonium  of  the  moss  was  regarded  as  representing 
the  form  from  which,  by  elaboration  of  vegetative  tissues 
and  organs,  the  sporophyte  of  the  fern  was  derived.  This 
view  was  clearly  expressed  in  1884  by  the  noted  botanist 
Nageli,  who  considered  that  the  sporophyte  of  Pterido- 
phytes was  derived  from  a  moss-like  sporogonium  by  the 
development  of  leafy  branches. 

A  consideration  of  the  fossil  record,  however,  makes  it 
difficult  to  accept  this  hypothesis.  Not  only  do  we  find, 
in  the  fossil  forms  described  above,  sporophytes  that  do 
not  bear  the  remotest  resemblance  to  the  moss-sporo- 
gonium,  but  fossil  mosses  and  liverworts  have  never  been 
positively  identified  in  either  the  Palaeozoic  or  the  Meso- 
zoic  rocks,1  while  the  same  rocks  are  rich  in  fossils  of  such 
advanced  forms  as  the  broad-leaved  sporophytes  of  the 
Cycadofilicales  and  Cycadophytes.  We  must  not,  how- 
ever, hastily  conclude,  from  this  lack  of  evidence,  that 
mosses  and  liverworts  did  not  exist  in  those  early  ages. 
Quite  possibly  they  were  present  when  the  Paleozoic  rocks 
were  being  deposited,  though  doubtless  not  represented  by 
the  same  genera,  or  at  least  not  by  the  same  species,  as 
are  now  living. 

lCf.,  however,  p.  172. 


236  HEREDITY  AND   EVOLUTION   IN   PLANTS 

162.  Summary  of  Results. — From  what  has  been  said, 
in  this  and  in  Chapter  VI,  we  recognize  that  the  method 
of  evolution  is  to  be  ascertained  chiefly  by  experiment — by 
studying  living  plants  in  action;  but  the  course  of  evolution 
chiefly  by  the  study  of  comparative  morphology,  with  special 
attention  to  fossil  forms,  and  supplemented  by  the  facts  of 
geographical  distribution.  Other  points  are  necessary  to 
complete  the  history  of  the  evolution  of  plants;  the  above 
paragraphs  give  only  the  barest  outline  of  the  problem, 
for  the  entire  history  is  much  too  long  and  much  too 
difficult  to  be  treated  here.  To  summarize;  the  facts  now 
known  have  led  some  investigators  to  infer: 

1.  The  origin  of  Angiosperms  from  Cycadophyta  (pro- 
angiosperms). 

2.  The  origin  of  Cycadophyta  from  Cycadonlicales. 

3.  The  origin  of  Cycadofilicales  from  Primonlices. x 

4.  The  origin  of  Filicales  from  Primonlices. 

5.  The  origin  of  Cordaitales  from  Primonlices. 

6.  The    origin    of    Coniferales  from  Cordaitales. 

An  ancestral  tree  embodying  these  views  is  shown  in 
Fig.  no. 

What  was  the  origin  of  the  Primonlices?  Here,  as 
often  in  every  science,  we  have  to  acknowledge  that 
we  do  not  know;  the  group  is  a  hypothetical  one,  and 
some  investigators  doubt  its  actual  existence  altogether. 

153.  Other  Views. — (a]  Other  and  equally  competent 
students  of  the  problem  take  exception  to  one  or  more 
of  the  six  points  tabulated  above.  Not  all  of  their  views 
can  here  be  discussed,  but  mention  may  be  made  of 
that  first  elaborated  by  Jeffrey,  of  Harvard  University. 

1  The  term  Primofilices,  not  hitherto  used  in  this  text,  refers  to  a  hypo- 
thetical, primitive  fern  stock. 


THE   EVOLUTION   OF   PLANTS 


237 


/Westers  of  Primo-filices 

FIG.  no. — Genealogical  tree,  showing  the  ancestral  lines  of  the  mod- 
ern plant  orders,  according  to  a  monophyletic  hypothesis.  Full  ex- 
planation in  the  text.  (Cf.  Fig.  in.) 


238  HEREDITY   AND    EVOLUTION   IN   PLANTS 

According  to  this  view,  vascular  plants  appear  at  the 
beginning  of  the  fossil  record  as  two  distinct  series,  the 
Lycopsida  and  Pteropsida.  The  Lycopsida,  like  the 
modern  Lycopodiales,  are  characterized  by  the  possession 
of  small  leaves  (a  primitive  character),  and  by  few  spor- 
angia on  the  upper  surface  of  the  leaves.  The  Pteropsida, 
by  contrast,  like  the  modern  Filicales,  are  in  general,  dis- 
tinguished by  large  leaves,  having  the  numerous  sporangia 
on  the  lower  surface.  The  two  groups  also  have  well- 
marked  anatomical  differences.  The  Lycopsida  reached 
their  greatest  development  in  the  Paleozoic  period,  and 
now  appear  to  be  on  their  way  to  extinction.  The  Pterop- 
sida, on  the  other  hand,  although  possessing  many  repre- 
sentatives in  former  geological  ages,  still  maintain  their 
full  vigor,  and  are  considered  by  this  school  of  paleo- 
botanists  to  be  in  the  direct  ancestral  line  of  our  modern 
vascular  plants,  substantially  as  indicated  in  Fig.  1 1 1 . l 

(b)  Greater  precaution  in  drawing  conclusions  from  the 
few  known  facts  has  led  still  other  students  of  fossil  plants 
to  refrain  from  endeavoring  to  connect  up  the  ancestral 
lines,  claiming  that  while  they  may  converge,  indicating  a 
common  ancestry  of  the  known  forms  in  the  geologic  past, 
on  the  other  hand  they  may  not  unite,  or  at  least  may  not 
all  converge  toward  the  same  ancestral  type.  In  other 
words,  it  is  suggested  that  fossil  and  modern  plants  had  a 
polygenetic  origin  from  the  stage  of  primitive  protoplasm. 
Such  views  are  illustrated  in  Table  V  (p.  240). 

It  is  seen  from  this  diagram  that  our  modern  ferns  have 
a  long  ancestral  history,  extending  from  the  present  back 

1  Scott  restricts  the  name  Lycopsida  to  the  Lycopodiales,  and  proposes 
a  third  group,  Sphenopsida,  incuding  the  Equisetales,  Pseudoborniales, 
Sphenophyllales,  and  Psilotales.  Wieland  has  recently  adduced  reasons 
for  using  the  term  Hemicycadales.  vs.  Bennettitales.  (Cf.  foot-note, 
p.  211.) 


THE   EVOLUTION   OF   PLANTS 


239 


_/)  rchigymnospermAe 
(Hypothetical) 


Ancestors  of 

FIG.  in. — Genealogical  tree,  showing  the  ancestral  line  of  the  modern 
plant  orders  according  to  a  polyphyletic  hypothesis.  Full  explanation 
in  the  text.  (Cf.  Fig.  no.) 


240 


HEREDITY   AND    EVOLUTION   IN   PLANTS 


to  early  Palaeozoic  times;  the  same  is  true  of  our  modern 
cycads,  maidenhair  tree  (Ginkgo),  club-mosses  (Lyco- 
podiales),  and  horse-tails  (Equisetales).  The  Coniferales 
may  be  traced  back  into  the  upper  Carboniferous  period, 
while  the  most  highly  developed  of  modern  plants,  the 
Angiosperms,  appear  to  have  come  into  existence  as  late 
as  about  the  middle  of  the  Mesozoic  era,  perchance  as 

TABLE  V 


| 

Ascendancy 

Periods 

Persistence  and  relationship  of 
great  groups 

Tertiary 

i 

8  iL 

s 

VII.  Reign  of  Angiosperms 

Cretaceous 
Comanchian 

I 

1 

llfl 

•a 

I 

~a 

Jurassic 

§ 

VI.  Reign  of  Pro-angiosperms 

Triassic 

O 

i 

w 

JJ 

< 

Permian 

1 

! 

u 

... 

3 

1 

I 

1 

i 

1 

! 

-j 

V.  Reign  of  Acrogens   (High- 
er  Equisetes.    Lycopods, 

Pennsylvanian 
Mississippian 

etc.) 

"£ 

>. 

"n 

o* 

^ 

;^ 

| 

U 

r/^ 

u 

^ 

;Ph 

IV.  Reign  of  Gymnospertns 

Devonian 

1 

0 

Silurian 

5.  Actual    Fossil 

Land 

Plant   rec- 

III.  Reign    of    Early    Land 

ord  begins 
4.  Primofilices  —  Early 

Equisetes 

Ordovician 

3-  B 

asal  Plant    Complex   with   va- 

r  ety  of  species 

Cambrian 

2.   Differential 

ion 

of 

Dry     Land 

II.  Reign  of  Algze 

Precambriap. 

ar 

d  Aqua 

tic  Plants 

(Proterozoic) 

(Fossil  Algae  a 

hu 

ndant) 

I.  Reign  -of    Primitive    Life 
(Hypothetical) 

Old  P  r  e  c  a  m- 
brian 
(Archeozoic) 

(Fossil  Algae  begin) 
i.  Primitive     Protoplasm    and 
Unicellular  L  fe 

In  the  above  table  (after  Wieland),  the  groups  are  to  be  considered  as 
arranged  on  an  unrolled  cylinder,  projected  from  a  hemisphere;  thus  the 
phyletic  lines  are  to  be  pictured  as  converging  below  toward  the  pole, 
and  the  Cordaitales  as  coming  between  the  Ginkgoales  and  Filicales,  to 
both  of  which  they  are  related. 


THE   EVOLUTION   OF   PLANTS  241 

recently  as  20  million  years  ago.  The  lateness  of  their 
appearance  and  the  rapidity  with  which  they  have  spread, 
until  they  are  now  the  dominant  element  in  the  flora 
of  the  land,  indicate  how  well  they  are  adapted  to  their 
environment.  "Nothing  is  more  extraordinary  in  the 
history  of  the  vegetable  kingdom,"  wrote  Darwin  to 
Hooker,  "than  the  apparently  very  sudden  or  abrupt 
development  of  the  higher  plants." 

"The  construction  of  a  pedigree  of  the  Vegetable  King- 
dom is  a  pious  desire,  which  will  certainly  not  be  realized 
in  our  time;  all  we  can  hope  to  do  is  to  make  some  very 
small  contributions  to  the  work.  Yet  we  may  at  least 
gather  up  some  fragments  from  past  chapters  in  the  history 
of  plants,  and  extend  our  view  beyond  the  narrow  limits 
of  the  present  epoch,  for  the  flora  now  living  is  after  all 
nothing  but  one  particular  stage  in  the  evolution  of  the 
Vegetable  Kingdom."1 

154.  The  Element  of  Geological  Time. — How  many 
years  has  it  taken  for  the  evolution  of  the  higher  Angio- 
sperms — that  is,  from  the  dawn  of  the  fossil  record  in  the 
Silurian  period  to  the  present?  No  one  knows.  From  a 
study  of  the  thickness  of  rock  strata,  and  a  knowledge  of 
the  probable  time  required  for  the  depositing  of  those 
strata  as  sediment  on  the  floor  of  the  ancient  oceans,  and 
their  elevation  and  denudation  to  their  present  condition 
by  weathering  and  erosion,  geologists  have  been  able  to 
suggest  relative  measures  of  geologic  time.  Paleozoic 
time  is  long,  twice  as  long  as  Mesozoic  time,  and  Meso- 
zoic  time  must  be  at  least  twice  as  long  as  Cenozoic  time. 
The  actual  age  of  the  earth  is,  however,  a  problem  which 
engages  the  attention  of  physicists  as  well  as  geologists. 

1  Scott,  D.  H.    "Studies  in  Fossil  Botany,"  p.  3. 


242  HEREDITY   AND   EVOLUTION   IN   PLANTS 

Sixty  years  ago  Lord  Kelvin  gave  a  mean  estimate  of 
100,000,000  years.  With  this  estimate  two  geologists, 
Walcott  and  Geikie,  have  nearly  concurred;  but  since  the 
discovery  of  radium  it  has  been  estimated  that  certain 
carboniferous  iron  ores  have  an  age  of  140,000,000  years. 
Figures  of  such  magnitude  convey  but  little  meaning  to 
our  minds;  they  are  too  large  for  us  to  grasp  their  real 
value.  " Therefore,"  as  Darwin  has  said,  "a  man  should 
examine  for  himself  the  great  piles  of  superimposed  strata, 
and  watch  the  rivulets  bringing  down  mud,  and  the  waves 
wearing  away  the  sea-cliffs,  in  order  to  comprehend  some- 
thing about  the  duration  of  past  time,  the  monuments  of 
which  we  see  all  around  us." 


CHAPTER  XIII 
THE  GREAT  GROUPS  OF  PLANTS 

155.  The  entire  question  of  taxonomic  groups  is  very 
difficult  and  intricate,  and  there  is  at  present  a  consider- 
able difference  in  opinion  and  usage,  even  among  those 
equally  competent  to  judge.  As  set  forth  in  Chapter  IX, 
the  segregation  and  sequence  of  larger  groups  may  be 
based  chiefly  upon  the  morphology  of  living  plants,  or 
upon  that  basis  supplemented  by  the  findings  of  anatomy 
(including  embryology  and  histology),  comparative  life 
histories,  and  paleobotany.1  Manuals  and  "Floras"  of 
systematic  botany  are,  for  the  most  part,  arranged  upon 
the  former  basis,  which  operates  at  present  in  the  direction 
of  conservatism  and  few  changes  in  connection  with  the 
largest  groups,  or  phyla.  Regard  for  the  evidence  from 
other  sources  is  more  apt  to  result  in  conflict  of  opinion  and 
more  frequent  revisions  in  the  light  of  new  studies,  but  it 
is  also  more  apt  to  result  in  a  closer  approximation  to  the 
truth.  In  the  former  case  the  sequence  of  groups  is 
chiefly  based  upon  complexity  of  organization,  proceeding 
from  the  simpler  to  the  more  complex.  On  this  basis  the 
monocotyledons,  for  example,  would  precede  the  dicotyle- 
dons, the  order  observed  in  the  Manuals. 

In  the  latter  case  the  sequence  of  groups  attempts  to 
indicate  or  reflect  their  order  of  development  in  time,  as 
indicated  by  the  data  of  paleobotany,  comparative  life- 
histories,  comparative  anatomy,  and  plant  geography. 
On  this  basis  the  monocotyledons  would  follow  the 

^ee,  also,  p.  236. 

243 


244  HEREDITY  AND   EVOLUTION  IN   PLANTS 

dicotyledons,  as  being  derived  from  the  latter  by  a  process 
of  simplification  (Cf.  p.  223).  The  structural  and  ana- 
tomical evidence  that  eu^porangiate  ferns  are  more 
ancient  than  legtosporangiate  ferns  is  rendered  more  cer- 
tain by  the  fact  that  the  earliest  fern  fossils  (in  Paleozoic 
rocks)  are  eusporangiate ;  the  leptosporangiate  forms  do 
not  appear  until  later,  and  the  fossils  belong  to  families 
closely  related  to  the  more  ancient  eusporangiate  group, 
while  the  fossils  of  more  recent  rocks  show  closer  affinities 
with  the  modern  living  forms.  (See,  however,  p.  30.) 

In  any  tabular  arrangement  including  all  the  great 
groups  or  phyla  every  group  must,  of  course,  come  defi- 
nitely after  some  one  group  and  precede  another.  Thus, 
mosses  logically  fall  between  the  Thallophytes  and  the 
fern  allies;  but  there  is  scarcely  any  evidence  that  they  are 
phylogenetically  related  to  the  groups  that  follow  them 
in  the  table.  Strangely  enough,  there  are  few  well- 
authenticated  fossil  remains  of  mosses  (and  those  not 
below  the  Mesozoic),  and  it  has  even  been  seriously 
suggested  that  they  may  have  developed  from  more 
complex  groups  by  processes  of  reduction  and  simplifica- 
tions; but  there  is  little,  if  any,  evidence  to  indicate  from 
what  higher  group  they  might  have  been  thus  derived, 
and  the  positive,  though  meager,  fossil  evidence  is  suffi- 
cient to  render  highly  improbable,  if  not  to  nullify,  the 
suggestion  of  derivation  by  reduction. 

The  old  group  "Pteridophytes,"  of  the  manuals, 
including  the  true  ferns  and  their  "allies"  (horsetails, 
lycopods,  and  little  club-mosses),  served  a  useful  purpose 
before  the  recent  researches  in  fossil  botany;  but  the 
results  of  those  studies  have  made  it  impossible  consist- 
ently to  maintain  the  group  longer  in  its  former  con- 


THE   GREAT  GROUPS   OF  PLANTS  245 

notation.  The  term  "  Pteridophy ta "  may  still  be  used 
to  advantage  in  a  more  restricted  sense,  as  applying 
to  the  "true  ferns,"  while  the  "fern  allies"  naturally 
fall  into  two  other  Divisions  or  phyla,  namely  the  Club- 
mosses  (Lepidophyta)  and  the  Horsetails  (Calamophyta 
of  Bessey,  or  Arthrophyta  of  Berry). 

The  discovery  of  the  fossil  seed-bearing  ferns  (Cycado- 
filicales)  and  their  fossil  and  living  relatives  (Hemicycad- 
ales,  Cycadales,  Cordaitales,  and  Ginkgoales),  all  having 
cryptogamic  (i.e.,  centripetal)  wood,1  and  all  the  living 
forms  distinguished  from  the  other  gymnosperns  by  the 
possession  of  ciliated  motile  sperms,  suggested  the  group 
to  which  Jeffrey  has  given  the  convenient  and  descriptive 
term  Archigymnosperma  (Early  Gymnosperms) :  in  contrast 
to  the  Yews  (Taxales),  Conifers  (Pines,  Spruces,  Hem- 
locks, Firs,  Cypress,  etc.),  and  Gnetales,  which  lack  both 
those  characters.  To  this  latter  group  Jeffrey  has  given 
the  name,  Metagymnospermce  (Late  Gymnosperms). 

Other  authors  have  suggested  grouping  the  woody- 
stemmed  and  comparatively  small  leaved  Cordaitales, 
Ginkgoales,  and  Coniferales  together,  and  apart  from 

1  The  first  formed  woody  tissue  is  primary  wood  or  protoxylem.  It  is 
present  when  the  organ  (stem,  root,  etc.)  is  young,  and  its  cell  walls 
are  thickened  in  rings  or  spirals  and  thus  it  can  readily  stretch  as  the 
organ  elongates  in  growth.  After  growth  in  length  has  ceased,  or  has 
been  greatly  retarded,  secondary  wood  or  melaxylem,  forms.  The  cell 
walls  of  this  tissue  have  scalariform,  reticulate,  or  pitted  thickening, 
and  thus  they  cannot  readily  stretch.  In  the  vascular  cryptogams  (e.g., 
Club-mosses  and  related  forms)  the  secondary  wood  forms  inside  the 
zone  of  primary  wood;  in  the  later  or  "higher"  gymnosperms  (Metagym- 
nosperms)  this  order  of  development  is  reversed;  while  in  the  ferns  and 
lower  gymnospenns  (Archigymncs perms)  the  earlier  development  is  cen- 
tripetal and  the  later  centrifugal.  Thus  the  mode  of  development  of 
the  woody  tissue  is  an  index  of  the  evolutionary  position  of  a  given 
form. 


246  HEREDITY   AND   EVOLUTION   IN  PLANTS 

the  Cycadean  series  (Cycadales,  Hemicycadales  (Bennet- 
titales) ,  Cycadofilicales)  which  have  pithy  stems ;  but  some 
of  the  Cordaitean  forms  also  have  pithy  stems  and  com- 
paratively large  leaves.  Here  again,  as  so  often,  an 
attempt  at  a  formal  classification  necessitates  drawing 
an  apparently  sharp  line  where  in  fact  one  does  not  exist. 
As  Professor  Jeffrey1  has  said,  the  term  Archigymno- 
spermae  is  one  of  convenience,  and  like  most  scientific 
terms  falls  short  of  covering  the  situation.  On  the  basis 
of  certain  criteria  (e.g.,  the  structure  of  the  wood),  the 
Ginkgoales  appear  to  be  intermediate  between  the  Coni- 
f erales  and  the  Cordaitales.  In  fact,  as  Jeffrey2  has  pointed 
out,  the  "living  fossil,"  Ginkgo,  may  be  regarded  as  a 
(connecting  h'nk  or  transitional  form  between  the  Archi- 
'gymnospermae  and  the  Metagymnospermae. 

The  relationship  of  Isoetes  is  one  of  the  most  difficult 
to  determine  among  all  the  vascular  cryptogams.  Argu- 
ments for  and  against  interpreting  it  as  derived  by  re- 
duction from  the  Lepidodendron  group  are  given  by  Lady 
Isabel  Browne.3  The  secondary  growth  in  thickness  of 
its  stem  (in  such  a  dwarfed  form)  must  be  regarded  as  a 
character  of  long  standing,  not  recently  acquired;  plants 
in  both  groups  have  mucilage  cavities.  Isoetes  resembles 
some  of  the  Lepidodendrales  (e.g.,  the  so-called  Stigmaria*) 
in  the  dichotomous  branching  of  its  roots.  Other  facts 
of  structure  (e.g.,  the  occurrence  of  the  sporangia  on  the 
upper  side  of  the  leaves)  have  also  been  interpreted 

1  Jeffrey,  E.  C      Science,  N.  S.  47:  316.     1918. 

2  Jeffrey,  E.  C.     The  anatomy  of  woody  ptanls,  p.  315.  Chicago,  1917. 
3 Browne,  Lady  Isabel.     The  phylogeny  and  inter-relationships  of  the 

Pteridophyta.    New  Phytologist  7 :    93,  103,  150,  181,  230.    1908. 

4  The  fossil  remains  to  which  the  generic  name  Stigmaria  was  assigned 
have  long  been  known  to  be  the  root-system  of  Sigillaria. 


THE    GREAT   GROUPS   OF   PLANTS  247 

as  pointing  to  the  origin  of  Isoetes  (by  reduction)  from 
the  Lepidodendrales.  One  of  the  most  cogent  objections 
to  this  theory  is  the  great  amount  of  reduction  which 
must  be  postulated;  moreover,  Isoetes  has  no  cone, 
while  most  of  the  Lepidodendrales  have.  The  absence 
of  secondary  growth  in  thickness  of  the  stem  and  of  a  ligule 
on  the  leaves,  combined  with  the  possession  of  a  biciliate 
sperm,  in  Lycopodium,  would  tend  to  preclude  its  close 
affinity  with  Isoetes .  While  certain  features  of  sporophy te 
anatomy  (e.g..  the  possession  >of  a  ligule)  suggest  Selag- 
inella,  it  seems  difficult  to  accept  a  close  relationship 
between  Isoetes  and  the  Selaginellales,  since  the  sperms 
of  the  latter  like  those  of  Lycopodium  are  biciliate,  while 
those  of  Isoetes  are  multiciliate.  The  possession  of  multi- 
ciliate,  sperms  and  the  structure  of  the  archegonia  suggest 
affinity  with  the  eusporangiate  pteridophytes,  and  notably 
with  the  Marattiales. 

Without  going  further  into  details  which  belong  to  a 
more  advanced  and  technical  treatise  than  this,  and 
disregarding  certain  mooted  points,  or  almost  equally 
balanced  choices  like  the  one  just  mentioned,  it  may  be  said 
that  the  following  tabular  statement  (pp.  249-251)  reflects 
the  present  state  of  our  knowledge  concerning  the  rela- 
tionship and  developmental  sequence  (phylogeny)  of  the 
great  Divisions  and  Orders1  of  the  Kingdom  of  Plants. 
The  same  thing  is  shown  diagrammatically  in  Fig.  112 
(p.  248).  The  tabular  statement  aims,  not  only  to  indi- 
cate the  relationship  and  sequence  of  groups,  but  also  to 
help  the  student  define  the  terms  commonly  met  with  in 
the  established  literature  of  botany. 

1  Attention  is  called,  in  passing,  to  the  uniform  termination  (-ales)  of 
the  plant  Orders. 


248 


HEREDITY   AND   EVOLUTION   IN   PLANTS 


CLUfiVOSSBS 

EOHSSIAILS 

HflN 

SEED   PLAHT3 

(lepido- 
phyta) 

(Calaao- 
phyta) 

(Pterldophyta 
In  restricted 

(Spermatophyta) 

sense) 

Lower 

Higher 

Anglo- 

Gynno- 

Oyamo- 

sperms 

sperms 

sperme 

I 

Hi 

j 

i 

I 

]  j 

a 

1 

0} 

1  1! 

E 

E 

s 

n 

1 

s  : 

3  8 

s  s 

S  S' 

1  1 

P 

i 

'!** 

( 

I 

I 

I 

1 
s 

| 

I 

>   c 
c 

1 

a 

g  g 

a  8 

J» 

l 

°  1 

£S 

O 

• 

3 

Seed 

8 

i 

0 

0 

nake 

1 

i 

« 

^ 

sper 

m  a 

i 

s 

o 

oi  1 

iated 

i 

i   ! 

E 

* 

0 

i 

I 

i 

Seeds 

o 

CO 

enolosed 

sperms 

S  S 

not 
ciliated 

£ 

1 

1 

•H   S) 

Seed's         /ha 
nake'd       /   gl 
.perl  ma      /   * 

S 

11 

ciliated    /     S  • 

i 

3  S 

•a 

i 

33s 

• 

3 

5 

/    Is 

i 

S  o  5 

i 

/    5 

i 

i  Hi 

1 

/       3 

I 

11 

| 

/  i 

Stem 
smooth, 

Stem  with 
ridges  & 

Euspor- 
angiate 

Lepto- 
sporan- 

Busporan-  /                g 
giate    / 

eaves 

JolntB, 

glate 

\ 

pirally 

leaves 

With   / 

\ 

r  ranged, 

whorled  , 

Without  seed* 

seeds  /                   n 

V 

porangia 

sporangia 

/                     0 

Ingle 

several 

/'          Legend:        * 

Leaves  small, 
sporangia  on 
upper  side , 
woody  cylinder 
ontlnuous 


Leaves  large, 
sporangia  on 
under  side, 
woody  cylinde 
with  foliar 
gaps 

PTEEOPSIDA 


\  living  plants 

|  well  known  fossil  plants 

|  fossils  of  doubtful  affinity 

+  exclusively  fossil  groups 


k  A  I  H  GROUPS  OF  VASCUIAH  PLAITS 
Their  apparent  affinities  and  approilnate  geological  distribution 

FIG.  112. 


THE    GREAT   GROUPS   OF   PLANTS  249 

TABLE  VI 
THE  GREAT  GROUPS  OF  THE  KINGDOM  OF  PLANTS 

MAIN  GROUPS  OF  NON-VASCULAR  PLANTS 
Plants  without  "flowers" — CRYPTOGAMS  (Nos.  1-5) 
Plant  body  usually  a  thallus;  sexual  organs  usually  one 

celled Thallophytes  i 

No  archegonia 

Chlorophyll-bearing Alga  10 

Non-chlorophyll  bearing Fungi  ib 

Plant  body  thalloid  or  leafy;  sexual  organs  usually  several 

celled Bryophytes  2 

Archegonia 
Protonema  rudimentary  or  wanting, 

sporophytes  with  elaters Liverworts  20, 

Protonema  well  denned, 

sporophytes  without  elaters Mosses  ib 

1.  Thallophytes 

10.  Alga 

Cyanophyceae  (Blue-green)      Phaeophyceae  (Brown) 
Chlorophyceae  (Green)  Rhodophyceae  (Red) 

i  b.  Fungi 

Myxomycetes  (Slime-molds)    Basidiomycetes  (Spores  on  stalks) 
Schizomycetes  (Bacteria)  Including  the  Basidiolichenes 

Phycomycetes  (Molds)  Fungi   imperfecti    (Life   histories 

Ascomycetes  (Spores  in  sacs)      imperfectly  known). 
Including  most  Lichens 
(Ascolichenes) 

2-66,  Archegoniates;  2-6c,  Embryophyta 

2.  Bryophytes 
20.  Liverworts 

Ricciales  Jungermanniales 

Marchantiales  Anthocerotales 

2b.  Mosses 

Spagnales  (Peat  mosses)          Bryales  (True  mosses) 
Andreaeales  (Black  mosses) 

MAIN  GROUPS  OF  VASCULAR  PLANTS 
Woody  cylinder  continuous,  (i.e.,  without  foliar  gaps), 
leaves  small,  sporangia  above — LYCOPSIDA 


250  HEREDITY   AND   EVOLUTION   IN   PLANTS 

3-5,  Vascular  Cryptogams 

Stem  smooth,  leaves  spirally  arranged,  sporangia  single .  Clubmosses  3 
Stem  with  ridges  and  joints,  leaves  whorled,  sporangia 

several Horsetails  4 

Woody  cylinder  discontinuous   (i.e.,  with  foliar  gaps), 
leaves  large,  sporangia  below — PTEROPSIDA 

Without  seeds • Ferns  5 

With    seeds — SPERMATOPHYTA    (PHANEROGAMS) Seed  Plants  6 

Ovules  naked,  endosperm  formed  before  fertilization .  Gymnos perms  6 

Sperms  ciliated Early  Gymnosperms  6a 

Sperms  not  ciliated Late  Gymnosperms  66 

Ovules  enclosed,  endosperm  formed  after  fertilization  .  Angiosperms  6c 

3.  Clubmosses  (Lepidophyta  (Bessey)) 
Lycopodiales  Selaginellales 
Isoetales  (?)                                        Lepidodendrales  (Fossil) 
Psilotales 

4.  Horsetails  (Calamophyta  (Bessey),  Sphenopsida  (Scott),   Arlhrophyla 

(Berry)) 

Spenophyllales  (Fossil)  Calamariales  (Fossil) 

Pseudoborniales  (Fossil)  Equisetales 

5.  Ferns  (Pteridophyta,  in  restricted  sense;  Filicinea). 
Eus  porangiatce  Leptosporangiata 

Primofilices  (Coenopterideas)          Osmundales 
Marattiales  Polypodiales 

Ophioglossales  Marsiliales 

(Isoetales?) 

6.  Seed-Plants 

(60  6*  66  GymnospermcB  of  Brongniart) 

6a.  Early  Gymnosperms  (Cycadophyta  (Nathorst)  except  Ginkgoales; 

Archigymnospermce  (Jeffrey)) 

Cycadofilicales  (Fossif)  Cordaitales  (Fossil) 

Hemicycadales  (Wieland)  =  Ginkgoales 

Bennettitales  of  Potonie"  (Fossil) 
Cycadales 

66.  Late  Gymnosperms  (Coniferae  (Hallier);  Metagymnospermae  (Jef- 
frey); SlrobilopJiyla  (Bessey)) 
Taxales  Finales 

Araucariales  Gne  tales 


THE  GREAT  GROUPS  OF  PLANTS  251 

6c.  Angiosperms  (Angiospermae;  Anthophyta  (Braun)) 
Two  cotyledons,  leaves  net-veined,  parts  of  the  flower  in  5*3  or  4*5 
— DICOTYLEDONS 
Apeiala  (petals  wanting) 

1.  Casuarinales  8.  Fagales 

2.  Piperales  9.  Urticales 

3.  Salicales  10.  Proteales 

4.  Myricales  n.  Santalales 

5.  Leitneriales  12.  Aristolochiales 

6.  Balanopsidales          13.  Polygonales 

7.  Juglandales  14.  Chenopodiales 

Polypelalce  (petals  distinct — wanting  in  a  few  exceptional  cases) 

1.  Ranales  8.  Mai  vales 

2.  Papaverales  9.  Pane  tales 

3.  Sarraceniales  10.  Opuntiales 

4.  Rosales  n.  Thymeliales 

5.  Geraniales  12.  Myr tales 

6.  Sapindales  13.  Umbellales 

7.  Rhamnales 

Sympetala;  Gamopetalce  (petals  more  or  less  united) 

1.  Ericales  6.  Plantaginales 

2.  Primulales  7.  Rubiales 

3.  Ebenales  8.  Valerianales 

4.  Gentianales  9.  Campanulales 

5.  Polemoniales 

One  cotyledon,  leaves  usually  parallel- veined,  parts  of  the  flower  in  3*3 
or  6's — MONOCOTYLEDONS 

1.  Naiadales  6.  Arales 

2.  Pandanales  7.  Xyridales 

3.  Graminales  8.  Liliales 

4.  Palmales  (Principes)  9.  Scitaminales 

5.  Cyclanthales  (SynanthaV)  10.  Orchidales 


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INDEX 


Acer  Pseudo-Plalanus,  222 
Acquired  characters,  7  2 
Acroperat  95 
Adianlum,  9,21,22 

concinnum,  25 

emarginatum,  126 
Africa,  145 

Agapanthus  umbeUatus,  229 
Agassiz,  Louis,  85 
Agassiz's  Hypothesis,  84 
Agathis,  232 

australis,  233 

Age  and  Area,  Hypothesis,  of,  176 
Aleurites  moluceara,  163 
Algae,  distribution  of,  176 
Alternation,  homologous,  135 

of  generations,  132 

ontogenetic  hypothesis  of,  136 
Ambrosia  crithmifolia,  149 
Anaphase,  36 

Anatomy,  evidence  from  compara- 
tive, 127 

results  of  the  method  of  com- 
parative, 129 
Andes,  149 
Angiopteris,  95,  203 

evecta,  204 

Angiosperm,  primitive,  225 
Angiosperms,  ancestors  of  the,  210 
Anisosorus  hir stilus,  I 
Annulus,  10 
Antheridia,  18,  22,  23 
Anthostrobilus,  232 
Apical  cell,  126 
Aralia,  156 
Archegoniates,  130 

17 


Archegonium,  18,  21,  24 

Archigymospserma,  245 

Arthro phyla,  245 

Ascaris,  38 

Ash, 113 

Aspidium  Filix-mas,  n 

Autochthonous  hypothesis,  142 

Azolla,  82 

Azores,  149,  150 

Bananas,  113 

Barberry  family,  171 

Beagle,  145 

Bean  pond,  186 

Beans,  66,  104,  105,  107 

Bennettitales,  211 

Bennettites,  214 

Berberidaceae,  171 

Bibliography,  252 

Biometry,  54 

Biophors,  75 

Birds,  dispersal  by,  149 

Blakeslee,  52 

Blue  eyes,  69 

Blue  rose,  73 

Bolivia,  149 

Boston  fern,  16,  109, 113 

Botany,  the  major  problem  of,  84 

Botrychium  Lnnaria,  128 

Boveri,  74 

Bowenia,  214 

serrulata,  203 
Bower,  Professor,  n 
Bracken  fern,  2 
Brake,  2 
Brassica  oleracea,  113,  124 


257 


INDEX 


Bread  mould,  80 

Broccoli,  112 

Brousonetia  papyrifera,  163 

Brown  eyes,  69 

Brown,  Lady  Isabel,  135 

Brussels  sprouts,  112 

Bryophytes,  127 

Bucholz,  127 

Bud-sport,  114 

Bulbils,  1 6 

Cabbage,  wild,  112 

Cacti,  thornless,  113 

C  alamo  phyla,  245 

Calla,  80 

Camptosorus  rhizophyllus,  !$»  17 

Canary  Islands,  161 

Cannabis  saliva,  163 

Capsella  Bursa-pasloris,  95 

Caslalia  flava,  179 

lelragona,  158 
Cauliflower,  112 
Ceylon,  145,  150 
Character-units,  64 

versus  unit-characters,  65 
Chromatin,  36,  75 
Chromosomes,  37 
Chrysanthemum  leucanthcmum,  154 
Cinnamon  fern,  12 
Classification,  121 

evolution  and,  122 
Clayton's  fern,  13 
Cliff -cabbage,  124 
Clover,  113 
Coal  balls.  204 
Cocos  Island,  143 
Coleoptile,  224 
Cone-flower,  114 
Cordailales,  233 
Cordailes,  232 
Corsica,  144 
Cotyledonary  ring,  222 


Cross-fertilization,  25 
Crossing,  increased  vigor  from,  69 
Crossotheca  Hoeninghausi,  202 
Cultivation,  escapes  from,  163 
Cultures)  pedigreed,  59 
Curve  of  frequency,  106 
Cycadeoidea,  212,  214,  220 

dacolensis,  208,  209,  210,  215, 
219 

Darloni,  216 

ingens,  215 

Wielandi,  213,  214 
Cycadocephalus  Sewardi,  208 
Cycadofilicales,  201,  207 
Cycas  circinalis,  212 
Cyrtomium  falcalum,  9 
Cyslopleris  bulb  if  era,  16 

Darwin,  Charles,  90,  91 
Darwinism,  90,  92 

mutation  theory  to,  118 
De  Candolle,  144 
Dendrobium  allenualum,  147 
Dendroceros.  174 
Determinants,  75 
Determiner,  64 
de  Vries,  Hugo,  74,  101,  102 
Diapensia  lapponica,  162 
Dicotyledons,     monocotyledonous, 

226 

Dicotyledony,  origin  of,  220 
Diclyota,  135,  137 

dicholoma,  136 
Diervilla,  154 

Lonicera,  155 

rimilaris,  155 

sessilifolia,  155 

Differentiation,  Dorso-ventral,   20 
Dionaeamuscipula,  178,  179 
Dioscorea  bonariensis,  226,  227 
Diplazium  zelanicum,  7 
Diploid,  25,  35 


INDEX 


259 


Disease-resistance,  breeding  for,  71 
Dispersal,  means  of,  142 
Distribution,  continuous,  168 

types  of,  154 

significance  of  geographical,  139 
Division,  heterotypic,  37 

homotypic,  37 
Dodder,  80 

Dominance,  law  of,  59 
Dominants,  63 
Dr  osera  filiformls,  197,  198 
Drynaria  meyeniana,  4 
Ditmortiera,  174 

Earthworms,  194 

Egg,  21,  24 

Elementary  species,  in 

Elephants,  94 

Elodea,  196 

Enation,  132,  135,  231 

Endemism,  164 

Environment,  adjustment  to,  41 

fitness  for,  93 

fitness  of  the,  83 

inheritance  and,  66 
Epiblast,  223,  224 
Epidermis,  4 

Equinoxes,  precession  of  the,  192 
Equisetum,  234 

telamateia,  126 
Eriocaulon  septangnlarc,  157 
Eugenia  malaccensis,  163 
Eugenics,  76 
Eupatorium,  156 
Evening-primrose,  114 
Evolution  and  classification,  122 
Evolution,  early  antagonism  to.  91 

experimental,  118 

inorganic,  82 

meaning  of,  79 

method  of,  84 

organic,  83 


Existence,  struggle  for,  42 
Experimental  evolution,  101 
Expression,  inheritance  versus,  40, 

48 
Extinction,  factors  of,  195 

Factor,  64 

Factors,  65 

Falkland  islands,  148 

Fern,  life-cycle  of  a,  35 

Fern,  life  history  of  a,  i,  20,  34 

Fertilization,  23,  24,  53 

Fertilization-membrane,  24 

Fimbristylis  spathacea  Roth,  146 

Fittest,  survival  of  the,  43,  97 

Fleabane,  164 

Foliage-leaf,  10 

Foot,  25 

Forbes,  Edward,  161 

Fossil,  what  is  a,  183 

Fossil-formation,  conditions  of,  185 

Fossil  record,  significance  of  the,  235 

Fronds,  4 

Gaertner,  120 

Gametes,  32 

Gametophyte,  33 

Gaps  in  the  fossil  record,  193 

Genealogical  tree,  237,  239 

Gene,  64,  65 

Generations,  alternation  of,  34 

two  kinds  of,  33 
Genoa,  144 

Geothallus  tuberostis,  174 
Germination,  17 
Germ-plasm,  75 
Ginkgo,  139,  232,  233,  246 

biloba,  168 
Ginkgoales,  246 
Glaciation,  effects  of  continental, 

1 60 
Gleichenia  circinata,  n 


260 


INDEX 


Gmelin,  141 

Grass-embryos,  224 

Gray,  Asa,  157 

Green  dahlias,  113 

Green  roses,  113 

Groups  of  plants,  the  great,  243 

Gymnosperms,  ancestors  of  the,  232 

early,  245 

late,  245 

Habit  of  life,  consequences  of  an 

amphibious,  130 
Half -race,  223 
Hamamelis,  156 
Haploid,  25,  35 
Hawaii,  145,  150 
Hawaiian  flora,  origin  of  the,  1 7  5 
Hawkweed,  76 
Haworthia  sp.,  49 
Hedge  mustard,  42 
Helix,  hortensis,  157 
H  enter  ocallis  fulva.  163 
Hemiangiosperma,  232 
Hemicycadales,  211,  238 
Hepatica,  128,  172,  174 
Heredity,  45 

experimental  study  of,  55 

.inheritance  versus,  50 

Johannsen's  conception  of,  67 
Heterozygous,  63 
Hibiscus  occuliroseus,  1 78 
Hieraceum,  76 

aurantiacum,  154 
Hittdebrandia  sandwicensis,  178 
Honey-locusts,  thornless,  113 
Humbolt,  140 
Hurricane  grass,  146 
Hybridizing,  artificial,  58 
Hydrangea,  156 
Hydrophytes,  196 

Idants,  75 
Idioplasm,  74 


Ws,  75 

Incrustation,  183 

Indian  corn,  69,  223 

Indian  pipe,  80 

indigenes,  177 

Indusium,  8 

Inheritance,  39,  92 

and  environment,  66 
and  reproduction,  50 
mechanism  of,  73 
versus  expression,  40 
versus  heredity,  50 
what  is,  46 

Isoetes,  246,  247 

Jack-in-the-pulpit,  80 
Jelly-fish,  194 

Johannsen,  40,  67 

Jordan,  120 

Kale,  112 

Kauri  pine,  233 

Keeling  Islands,  143 

Kerner,  95 

Kingdom  of  plants,  the  great 

groups  of  the,  249 
Knight,  120 
Kohlrabi,  112 
Kolreuter,  120 
Krakatoa,  146,  149,  175 

Labrador,  160 
Lagenostema  Lomaxi,  205 
Lakes,  filling  up  of,  187 
Lamarck,  Jean  Baptiste,  89 
Lamarckism,  arguments  against, 
Lamarck's  hypothesis,  86 
Landscape,  a  late  paleozoic,  234 
Lang,  136 
Layering,  32 
Leaf,  free-living,  81 
Leersia  clandestine^,  224 


INDEX 


26l 


Lemna,  80 

trisulca,  81 

Lepidodendron,  134,  246 
Lepidophyta,  245 
Leplochloa  arabica,  224 
Leptosporangiate  ferns,  10 
Lesczyc-Suminski,  Count,  20 
Life-cycle,  cytological,  38 
Life-histories,  evidence  from,  125 
Life  history,  i,  3 
Linin,  36 
Linkage,  119 
Linnaeus,  121 
Linnaea  borealis,  121 
Liquidambat,  156 
Liriodendron,  170,  219 
Lomaria  eriopust  207 
Lonicerajaponica,  163 
Loxsoma  Cunninghami.  1 1 
Lunularia  cruciata,  1 73 
Lycopersicum  esculentum,  51 
Lycopodium,  134,  247 

cotnplanatum,  128 

lucidnlum,  156 

Selago,  128 
Lycopods,  134 
Lycopsida,  238 
Lyginodendron  oldhamiutn,  201, 202, 

205 
Lygodium  japoricum,  1 1 

Macrozamia,  214 

Moorei,  176 

spiralis,  211 
Madder  family,  171 
Magnolia,  156,  218,  219 

flower  of,  217 
Magnoliacece,  219 
Maidenhair  fern,  25 
Maiosis,  37 
Maize,  60,  62 
Malay  archipelago,  143,  147,  151 


Malthus,  94 

Marattia  Douglasii,  126 

fraxinea,  206 
Marattiales,  247 
Marble,  188 
Marchanlia,  173 

polymorpha,  128 
Marchantiaceae,  173 
Marsllia  vestila,  126 
Matonla  pectinata,  n 
Megasporophytes,  129 
Membrane,  nuclear,  36 
Mendel,  Gregor,  55 
Mendelian  ratio,  significance  of  the, 

64 

Mendel,  investigations  since,  77 
Mendelism,  relation  of  Weismann- 

ism  to,  76 
Mendel's  discoveries,  59 

discoveries,  value  of,  68 

law,  applications  of,  66 

method,  56 

problem,  56 

Metagymnospermae,  245 
Metamorphism,  188 

contact,  1 88 

regional,  188 
Metaxylem,  245 
Microsporophytes,  129 
Migrations,  plant,  192 
Mistletoe,  80 

American,  96 

European,  96 
Mltchella,  172 

repens,  172 
Mitosis,  36 
Moluccas,  150 
Monoclea,  174 
Monocotyledon,      dicotyledonous, 

226,  227 
Monocotyledony,  225 

origin  of,  223 


262 


INDEX 


Monophyletic  hypothesis,  237 
Mortts  alba,  163 
Moss-roses,  113 
Mt.  Gedeh,  174 
Mt.  Washington,  158 
Mucor,  51 

hermaphroditic,  52 
Multiplication,  vegetative,  16 
Mutant,  in 

Mutation   and   discontinuous   dis- 
tribution, 1 66 

examples  of,  in 

Mutation    theory   to    Darwinism, 
Relation  of,  118 

value  of  the,  120 
Mutations,  no 

Nageli,  74,  76 
Naudin,  120 
Nelumbo,  171 

In  tea,  171 

nucifera,  171 
Nepenthes  ampullaria,  145 

phyllamorpha,  147 
Nephrodium  filix-mas,  95 
Nephrolepis,  16,  109 
Nice,  144 

Nicotiana  tabacum,  51 
Nolothylus  orbicularis,  126 
Nymphaeaceae,  171 
Nymphaea  mexicana,  179 

Oak,  103 

Objections,  difficulties  and,  97 

Oenothera  biennis,  1 15 

breustylis,  115,  116 

glgas,  117,  167 

laevifolia,  116,  167 

Lamarckiana,  109,  114 
115,  116,  167 

nanella,  109 

scintillans,  167 


Onoclea,  24,  156 
Ontogeny,  83,  136 

evidence    from    comparative, 

126 

Oosperm,  25,  33 
Oospore,  25 

Ophloglossum  petidulum,  126 
Orchid,  seed  capsule  and  seeds  of 

an,  172 

Orchidaceae,  171 
Organs,  origin  of  vegetative,  131 
Orton,  W.  A.,  72 
Osmunda,  169,  204 

Claytoniana,  5,  13,  126 

clnnamomea,  12,  154 

Japonica,  154 

regalis,  154 
Osmundacea,  139 

Paleobotany,  the  scope  of,  183 

Paleogeography,  190 

Palisade  layer,  8 

Pangens,  74 

Papaver  Rhoeas,  222 

Paulownia,  163 

Pea,  edible,  61 

Peak  of  Teneriffe,  161 

Pendulun,     illustration     of     the, 

no 

Petrifaction,  183 
Petrifactions,  185 
Phaseolus  vulgaris,  105 
Phleum  alpinum,  162 
Phoradendron  flavescenst  96 
Photosynthesis,  8 
Phy salts  Alkekengi,  50,  51 
Phycomycetes,  80 
Phyllitis,  9 
Phylogeny,  83 
Phymatodes,  7 
Pinguicula  caudata,  227 

vulgaris,  226,  227 


INDEX 


263 


Finns  Banksiana,  221 

Laricio,  126 

Slrobus,  53 
Pisum  sativum,  61 
Plantago  major,  95 
Plantain,  95 

Plant  groups,  sequence  of,  130 
Plants,  evolution  of,  124,  201 
Pleurococcus,  80,  127 

vulgaris,  46,  47 
Plover,  American  golden,  151 

Pacific  golden,  151 
Plitchea  faetida,  164 
Polydemics,  177 
Polyphyletic  hypothesis,  239 
PolypodiacecR,  139,  169 
Poly  podium,  7,  8,  9,  12 

pundatum,  6 

•venosum,  128 
Polysiphonia,  135 
Porretta,  128 
Potentilla  nhea,  i'6i 
Primofilices,  236 
Prince's  Island,  148 
Pritchardia,  165 
Proangiosperms,  216 
Pro-anthostrobilus,  232 
Problem,  the  modern,  100 
Propagation,  32 

vegetative,  49,  50 
Prothallus,  18,  19 
Protonema,  17,  18 
Protoxylem,  245 
Primus  Graves ii,  180 

maritima,  180 
Pteridophyta,  245 
Pteridophytes,  244 
Pteridosperms,  201 

significance  of  the,  206 
Pleris,  9 

aquilina,  2,  8,  154 

longifolia,  6 


Pteropsida,  238 

Puccinia  glumarum,  71 

Pure  line  breeding,  68 

Purity  of  gametes,  theory  of  •,  63,  64 

Quercus  chrysolepis,  103 
Quetelet,  105 
Quetelet's  curve,  106,  107 
Quetelet's  law,  105 

Race,  middle,  223 
Radium,  73,  242 
Ranales,  218 
Ranuncuhts  aquatilis,  86,  87 

Ficaria,  226 
Receptacle,  8 
Reduction,  35 

nature  and  method  of,  36 
Relict  endemic,  167 
Relicts,  177 
Reproduction,  inheritance  and,  50 

sexual,  32,  53 
Rhizoid,  18,  17,  23 
Rhizomes,  3 
Rhizophora  Mangle,  228 
Rhizopus,  51 
Rhododendron  lapponicum,  159 

verticillatum,  147 
Riccia,  126,  129,  173 

trichocarpa,  128 

Rock  strata,  classification  of,  189 
Rocks,  stratification  of,  188 
Root-stocks,  3 
RiibiacecE,  171,  172 
Riibus  chamaemorus,  155 
RudbecKia  sp.,  114 
Rust  disease,  71 

Salina,  199 

Saltation,  orthogenetic,  109 

Saliiinia,  82 

Sap,  nuclear,  36 


264 


INDEX 


Saprophytism,  80 

Sardinia,  144 

Sassafras,  170 

Scarlet  tanager,  151 

Sckizaea  pusilla,  155 

Schouw,  142 

Schouw's  hypothesis,  141 

Scutellum,  224 

Seed-bearing   ferns,    discovery   of, 

201 
Segregation,  law  of,  60 

Mendelian,  61,  62 

ratio  of,  61 
Selfing,  66 
Self-pruning,  32 
Semon,  143 
Senecio,  148 
Sequoia,  168,  169 

giganlea,  168,  178 

sempervirens,  168,  178 
Seychelles,  145 
Shepherd's  purse,  95 
Shield  fern,  n,  95 
Shirley  poppies,  113 
Sigittaria,  246 
Silent  odontlpetda,  222 
Siphonogam>,  133 
Skunk  cabbage,  80 
Slate,  188 
Small,  James,  147 
Snakes,  87 
Solanaceae,  171 
Solatium  integrifolium,  50,  51 

nlgrum,  50,  51 
Solomon  archipelago,  151 
Sorus,  8 

Spartina  cynosuroides,  224 
Special  creation,  doctrine  of,  79 
Spencer,  Herbert,  43,  97 
Spencerites,  134 
Spermatophytes,  130 
Sperms,  21,  22,  23 


Sphenophyllum,  184 
Sphenopsida,  238 
Sphenopteris  Hoeninghausi,  201 
Spiranihes  Romanzoffiana,  157 
Spirem,  36 
Spirogyra,  194,  196 
Sporangia,  8, 10,  1 1 
Sporangiophore,  8 
Spore-production,   consequence   of 

enormous,  131 
Spores,  dispersal  of,  17 

germination  of,  17,  18 

reproduction  by,  32,  50 
Sporophyll,  9 
Sporophylls,  6,  7,  8,  n 
Sporophyte,  33 

steps  in  the  evolution  of  the,  1 3  2 
Sporophytes,  8 
St.  Helena,  148 
Stangeria  eriopns,  207 

paradoxa,  207 
Sterilization,  14 

progressive,  126,  128 
Stigmaria,  246 
Stolons,  16,  17 
Strawberry,  113 
Strobiloid  theory,  218,  219 
Struggle  for  existence,  94,  96,  153 
Stumps,  Fossil  tree,  191 
Sunflowers,  red,  113 
Sweet  flag,  80 
Sweet  peas,  66 
Switzerland,  145 
Sykes,  Miss,  134 
Symmetry,  bilateral,  19 
Symplocarpus  fatidus,  155 
Synangia,  209 
Synapsis,  37 
Synizesis,  37 

Taal  volcano,  152 

Tamus  communis,  226,  227 


Targionia,  173 

Taxodium,  170 

Tectoria  cicutaria,  14,  16 

Teneriffe,  Peak  of,  161 

Tern,  arctic,  151 

Thallophytes,  130 

Thomson,  J.  Arthur,  48 

Thyrsopteris  elegans,  n 

Tierra  del  Fuego,  149 

Time,  distribution  of  plants  in,  192 
distribution  of  plants  in  geo- 
logic, 193 

table  of  geological,  190 
the  element  of  geological,  241 

Toad-stools,  80 

Todea  barbara,  n 

Transpiration,  8 

Trapa  natans,  226 

Tree  ferns,  3 

Tree,  hypothetical  ancestral,  137 

Treubia  insignis,  1 74 

Triticum  mlgare,  224 

Twin-flower,  121 

Unconformity,  189 
Unit-characters,        character-units 

versus,  65 
Urtica  dioica,  163 
Utricularia,  196 

V  allisneria,  196 
Variation,  41,  93 

and    inheritance,    fluctuating, 
108 

continuous,  103 

curves  of,  109 

discontinuous,  108 


INDEX  265 

Variation,  fluctuating,  103,  106 
illustrations  of  continuous,  104 
two  kinds  of,  103 

Variety,  constant,  223 
eversporting,  223 

Vascular  plants,  8 

Venus  fly  trap,  179 

Viscum  album,  96 

Vries,  Hugo  de,  74,  101,  102 


Walking  fern,  15,  17 

Wallace,  Alfred  Russell,  90,  166 

Wanakena,  186 

Warming,  145 

Water,  dispersal  by,  149 

Water  buttercup,  86,  87 

Water  lily  family,  171 

Weismannism,  74 

to  Mendelism,  Relation  of,  76 
West  Indies,  146 
White  Mountains,  160 
White  pine,  53 
Wides,  177 
Wielandiella,  220 
Wiesnerella  Javanica,  174 
Wittiamsonia,  214,  220 

mexicana,  219 
Willow,  32 

Wind,  dispersal  by,  143 
Wolffia  papulifera,  81 

punclata,  81 

Zamia,  215,  232 
Zea  Mays,  69,  223,  224 
Zelkowa,  167 
Zygospore,  51 
Zygote,  32,  33 


